Solar cell structures for improved current generation and collection

ABSTRACT

In one aspect, optoelectronic devices are described herein. In some implementations, an optoelectronic device comprises a photovoltaic cell. The photovoltaic cell comprises a space-charge region, a quasi-neutral region, and a low bandgap absorber region (LBAR) layer or an improved transport (IT) layer at least partially positioned in the quasi-neutral region of the cell.

STATEMENT REGARDING FEDERALLY SPONSORED RESEARCH

This disclosure was made with Government support under the Dual-UseScience and Technology (DUS&T) Program, Contract Nos. F29601-98-2-0207and FA9453-04-2-0042 awarded by the U.S. Air Force Research Laboratory(AFRL). The Government has certain rights in this disclosure.

FIELD

The present disclosure generally relates to optoelectronic devices, andmore specifically, to single junction and multijunction solar cells, tophotovoltaic cells having low band-gap absorber regions (LBARs), and tolattice-matched and metamorphic solar cells.

BACKGROUND

A photovoltaic device or solar cell is a device that is capable ofconverting sunlight to electrical energy by the photovoltaic effect. Asolar cell, such as a multijunction solar cell, can have one or morecomponent photovoltaic cells, also called subcells. These componentphotovoltaic cells, or subcells, may be connected in series to form amultijunction solar cell, but may also be connected in other electricalconfigurations, such as in parallel, or in a combination of series andparallel connections.

The interest in solar cells has been increasing due to concernsregarding pollution, energy security, and limited available resources.This interest has been for both terrestrial and space applications. Inspace applications, solar cells have been in use for more than 40 yearsand the development of higher efficiency solar cells enables increasedpayload capabilities. In terrestrial applications, higher solar cellefficiency for conversion of sunlight to electricity results in asmaller collecting area required for a given electrical power output,and therefore lower cost per watt, and greater cost effectiveness for aterrestrial photovoltaic system.

The cost per watt of electrical power generation capacity forphotovoltaic systems inhibits their widespread use in terrestrialapplications. The conversion efficiency of sunlight to electricity istypically of crucial importance for terrestrial PV systems, sinceincreased efficiency results in a reduction of all area-relatedelectricity generation system components (such as cell area, module orcollector area, support structures, and land area) for a required poweroutput of the system. For example, in concentrator solar cell systemswhich concentrate sunlight from around 2 to around 2000 times onto thesolar cell, an increase in efficiency typically results in a reductionof an area comprising expensive concentrating optics. Improvements insolar cell efficiency are extremely leveraging at the system level, andthe dollar per watt ($/watt) ratio is a typical figure-of-merit appliedat the system level. For satellites, solar panels represent <10% of theentire system cost so that a relative improvement in solar cellefficiency of 3% over an existing technology generation results inleveraged cost savings. The same is true of terrestrial concentratorsolar power systems where the cost of the solar receiver is a fractionof the overall system cost.

To increase the electrical power output of such cells, multiple subcellsor layers having different energy bandgaps have been stacked so thateach subcell or layer can absorb a different part of the wide energydistribution in the sunlight. This arrangement, called a multijunction(MJ) solar cell, is advantageous, since each photon absorbed in asubcell corresponds to one unit of charge that is collected at thesubcell operating voltage, which increases as the bandgap of thesemiconductor material of the subcell increases. Since the output poweris the product of voltage and current, an ideally efficient solar cellwould have a large number of subcells, each absorbing only photons ofenergy negligibly greater than its bandgap.

In multijunction solar cells it is often desirable to modify thebandgaps of the semiconductor layers that form the subcells within themultijunction cell, and thereby modify the subcell voltages andwavelength ranges over which the subcells respond to incident light, forinstance, to space and terrestrial solar spectra. The specific bandgapsand thicknesses of layers that form the subcells within a multijunctioncell determine the subcell voltages, the current densities of eachsubcell, whether the subcell current densities can be matched to oneanother as is desired in a series-interconnected multijunction cell, andhow the broad solar spectrum is divided into narrower wavelength rangesby the combination of subcell bandgaps to achieve highersunlight-to-electricity conversion. A crucial technological challenge inthe design of multijunction solar cells is how to achieve the optimum ornear-optimum combination of subcell layer bandgaps, and how to achievethe desired wavelength ranges of subcell response—the wavelength rangesin which the subcells have photogenerated current that can be collectedusefully—in order to maximize the multijunction solar cell efficiency.Often the semiconductors that are readily useable—e.g., semiconductorsthat are lattice-matched to relatively common, inexpensive substrates;that can be grown with favorable minority-carrier properties such aslifetime and mobility; or that do not cause unwanted doping orimpurities in other parts of the cell—do not have the bandgaps thatresult in the most favorable combination of multijunction subcellbandgaps for conversion of the solar spectrum.

In optimum subcell bandgap combinations for multijunction cells undertypical space (AM0) and terrestrial (AM1.5 Direct, or AM1.5D) solarspectra, the desired bandgap of the upper subcell—also referred to asthe top cell, or cell 1 (C1), also called subcell 1—is often greaterthan the bandgap of GaInP at the same lattice constant of cell 2 (C2)upon which the top cell is grown. Therefore, it is desirable to useAlGaInP to raise the bandgap of cell 1. However, Al-containingsemiconductors often have diminished minority-carrier properties inpractice, such as lifetime, mobility, and diffusion length, compared totheir Al-free counterparts, resulting in reduced current collection fromAl-containing solar cell layers. This is particularly evident when thetop cell emitter is formed from n-type AlGaInP. Thus there is a need forsolar cell layers with improved minority-carrier properties and currentcollection that can be used in a top cell for which the main absorberlayer is high-bandgap AlGaInP.

Additionally, in multijunction solar cells some photogenerationlayers—defined here as layers in which charge carriers arephotogenerated and can be collected, including in a useful manner—havelow absorptance due to other design constraints in the solar cell. It isdesirable to increase photogeneration in these weakly absorbingstructures, and thereby increase current density of the subcell andmultijunction cell.

Past approaches to increasing photogenerated current density includeincreasing the thickness of current generating regions for which thereis insufficient light absorption above the bandgap. However, in manycases, absorption of light by the solar cell with photon energy abovethe solar cell bandgap is nearly complete, so increasing the thicknesshas little effect on the current, or can cause the current to decreasedue to poorer collection of photogenerated charge carriers from thickersolar cell layers. Another approach has been to lower the bandgap of thesemiconductors used to form the current generating regions of a solarcell. However, this approach also lowers the solar cell voltage. Inaddition, lowering the bandgap by changing the semiconductor compositionoften changes the crystal lattice constant, creating a greater latticemismatch with other layers in the solar cell, which can lead to a higherdensity of harmful dislocations in the lattice-mismatched subcell.

There exists a need for solar cells and other optoelectronic deviceshaving 1) semiconductor layers that photogenerate a greater quantity ofcharge carriers; 2) semiconductor layers that facilitate collection ofphotogenerated minority charge carriers in the solar cell to form usefulcurrent density; 3) a more nearly optimum combination of subcellbandgaps and subcell wavelength response ranges in multijunction devicesfor better energy conversion efficiency; and 4) device structures thatincrease the photogeneration in and usefully collected current fromweakly absorbing or incompletely absorbing photogeneration layers in thesolar cell.

SUMMARY

The present disclosure provides device structures that, in someimplementations, 1) result in greater photogeneration of chargecarriers, 2) result in greater collected current density fromphotogenerated charge carriers, 3) result in more advantageouscombinations of bandgaps and wavelength response ranges in multijunctionsolar cells and other optoelectronic devices, and/or 4) enhance thecurrent density from weakly absorbing photogeneration layers. Thesedevice performance aspects can interact strongly with one another in asolar cell or other optoelectronic device, and the disclosed devicestructures may have one or more of these advantageous features. Thus, insome cases, the disclosed devices offer increased efficiency andperformance.

In some implementations, a device described herein is an inorganicsemiconductor device. Moreover, a device described herein can comprise aphotodiode or a photovoltaic device, such as a multijunctionphotovoltaic device.

In accordance with the disclosure, a semiconductor structure isdisclosed that, in some implementations, achieves greater photogeneratedcurrent density due to one or more low-bandgap absorber region (LBAR)layers positioned in part or entirely in the quasi-neutral region orregions of a solar cell. For reference purposes herein, a LBAR layer isan absorbing layer that has a lower bandgap than one or both of the twoimmediately adjacent semiconductor layers. The low bandgap of an LBARlayer may be achieved by various means, as detailed below. Additionally,for reference purposes herein, an immediately adjacent semiconductorlayer, in some implementations, is in contact or forms a junction withthe LBAR layer.

An absorbing layer, for reference purposes herein, comprises a layerthat absorbs incident electromagnetic radiation and generates anelectron-hole pair or extractable charge carriers as a result. Further,the quasi-neutral region, as understood by one of ordinary skill in theart, is a non-space-charge region. Thus, in some implementations, thequasi-neutral region comprises the region within the emitter layerand/or base layer of a semiconductor structure that is not thespace-charge region. Moreover, as understood by one of ordinary skill inthe art, the boundaries of a space-charge region of a semiconductorstructure can be affected by one or more of the semiconductor materialsused, the doping level of the materials, and the applied bias oropen-circuit voltage. In addition, for reference purposes herein, anemitter layer is in front of or above a base layer, such that theemitter layer is the photogeneration layer that is closer to theincident radiation.

In another aspect, a semiconductor structure is disclosed that, in someimplementations, achieves greater semiconductor device current density,voltage, fill factor, and/or energy conversion efficiency due to one ormore improved transport (IT) layer(s) positioned in part or entirely inthe quasi-neutral region or regions (non-space-charge region or regions)of a semiconductor device such as a solar cell or photodetector. Forreference purposes herein, an transport (IT) layer can be a layer withhigher minority-carrier lifetime, minority-carrier mobility,minority-carrier diffusion length, majority-carrier mobility,majority-carrier conductance, charge-carrier saturation velocity in anelectric field, and/or collected photogenerated current density than inthe primary photogeneration layers, one or more adjacent photogenerationlayers, a replacement layer having the same composition as one or moreadjacent photogeneration layers, or the bulk of the other semiconductorlayers of the semiconductor device. A replacement layer, for referencepurposes herein, comprises a hypothetical layer that can replace an ITlayer in a semiconductor structure. For example, in someimplementations, an IT layer has a higher photogenerated current densitythan an immediately adjacent photogeneration layer or a replacementlayer having the same composition as an immediately adjacentphotogeneration layer, such as a higher collected photogenerated currentdensity. In some implementations, an IT layer has a higherminority-carrier diffusion length than an immediately adjacentphotogeneration layer or a replacement layer having the same compositionas an immediately adjacent photogeneration layer. In someimplementations, an IT layer has a higher minority-carrier lifetimeand/or minority-carrier mobility than an immediately adjacentphotogeneration layer or a replacement layer having the same compositionas an immediately adjacent photogeneration layer. The improved transportproperties may be achieved by various means, as detailed below.

In another aspect, the low-bandgap absorber region (LBAR) layer(s)described herein may also be improved transport (IT) layer(s).Throughout this disclosure, described herein means described in thispatent disclosure.

In another aspect, the improved transport (IT) layer(s) described hereinmay also be low-bandgap absorber region (LBAR) layer(s).

Thus, in some cases, the LBAR layer and the IT layer of a devicedescribed herein are the same layer. Alternatively, in otherimplementations, a device comprises an LBAR layer and an IT layer,wherein the LBAR layer and the IT layer are different layers.

In another aspect, the one or more LBAR and/or IT layers are positionedpartly in a quasi-neutral region of a solar cell, and partly in aspace-charge region. In another aspect, the one or more LBAR and/or ITlayers are positioned entirely in a quasi-neutral region of a solarcell. In another aspect, the one or more LBAR and/or IT layers arepositioned in part or entirely in the quasi-neutral region of theemitter of a solar cell. In another aspect, the one or more LBAR and/orIT layers are positioned in part or entirely in the quasi-neutral regionof the base of a solar cell.

In another aspect, the one or more LBAR and/or IT layers form theentirety of a functional layer within a solar cell. For referencepurposes herein, a functional layer of a cell can comprise a layer thatgenerates charge carriers or facilitates the extraction of chargecarriers. For example, in some implementations, a functional layercomprises an emitter layer, base layer, window layer, orback-surface-field (BSF) layer.

In another aspect, the one or more LBAR and/or IT layers are positionedin the solar cell emitter in one of the following configurations: partlyin the emitter quasi-neutral region and partly in the emitterspace-charge region; entirely in the emitter quasi-neutral region; orentirely in the emitter space-charge region. The LBAR thickness mayrange from 0% to 100% of the total emitter thickness, and may be variedto affect current balance in a multijunction cell.

In another aspect, the one or more LBAR and/or IT layers form the entireemitter layer of a solar cell, and are positioned partly in the emitterquasi-neutral region and partly in the emitter space-charge region. Forexample, in some implementations, a cell further comprises a windowlayer, a BSF layer, a base layer, and an emitter layer, and the LBARlayer or IT layer forms the entirety of an emitter layer and the baselayer and the emitter layer form a photoabsorber layer disposed betweenthe window layer and the BSF layer.

In another aspect, the one or more LBAR and/or IT layers are positionedpartly in the emitter quasi-neutral region and partly in the emitterspace-charge region, are positioned such that at least one LBAR and/orIT layers is in contact with the base layer of a given base doping type(n-type, p-type, or intrinsic), and are positioned such that the one ormore LBAR and/or IT layers are separated from the window layer by anadditional part of the emitter with different doping type from that ofthe base. For example, in some implementations, the LBAR layer or the ITlayer forms a first portion of an emitter layer of the cell and the cellfurther comprises a window layer; a back-surface-field (BSF) layer; abase layer; and a second portion of the emitter layer. The first portionof the emitter layer is disposed between the base layer and the secondportion of the emitter layer. The first portion of the emitter layer ispositioned partially in the space-charge region of the emitter layer andpartially in the quasi-neutral region of the emitter layer.

In another aspect, the one or more LBAR and/or IT layers are positionedpartly in the emitter quasi-neutral region and partly in the emitterspace-charge region, are separated from the base layer by part of theemitter, and are separated from the window layer by an additional partof the emitter.

In another aspect, the one or more LBAR and/or IT layers are positionedentirely in the emitter quasi-neutral region, are positioned such thatat least one LBAR and/or IT layer is in contact with the window layer,and are separated from the base layer by an additional part of theemitter. For example, in some implementations, the LBAR layer or the ITlayer forms a first portion of an emitter layer of the cell and the cellfurther comprises a window layer; a back-surface-field (BSF) layer; abase layer; and a second portion of the emitter layer. The secondportion of the emitter layer is disposed between the base layer and thefirst portion of the emitter layer. The first portion of the emitterlayer is adjacent the window layer. And the first portion of the emitterlayer is positioned entirely in the quasi-neutral region of the emitterlayer.

In another aspect, the one or more LBAR and/or IT layers are positionedentirely in the emitter quasi-neutral region, are separated from thebase layer by a first additional part of the emitter, and are separatedfrom the window layer by second additional part of the emitter.

In another aspect, the one or more LBAR and/or IT layers form the entireemitter region, in a solar cell having a base with higher bandgap and/orreduced thickness with respect to the emitter, such that the emitterlayer is a major photoabsorbing region of the solar cell, such that30-100%, and preferably 50-100%, of the photogeneration in the solarcell comes from the emitter layer.

In some implementations, the emitter layer has a thickness that isbetween about 50% and about 100% of the total thickness of thephotoabsorber of the cell. For reference purposes herein, thephotoabsorber of the cell, in some cases, consists of the emitter layerand any base layer that is present in the cell. It is also possible forthe emitter layer to be thinner than the base layer of the cell. Forexample, in some cases, the emitter layer has a thickness that is up toabout 20% or up to about 30% of the sum of the emitter layer thicknessplus the base layer thickness. In some implementations, the emitterlayer has a thickness that is between about 1% and about 20% of the sumof the emitter layer thickness plus the base layer thickness.

Moreover, an emitter comprising one or more LBAR and/or IT layers may bepartly in the solar cell quasi-neutral region, and partly in the solarcell space-charge region. The LBAR thickness may range from 5% to 100%of the total solar cell photoabsorber thickness, and may be varied toaffect current balance in a multijunction cell.

In another aspect, the one or more LBAR and/or IT layers form the entireemitter region of a solar cell, comprising a p-n junction between theemitter layer and a back surface field (BSF) layer with higher bandgapthan the emitter layer, and for which there is no base layer (zerothickness base layer) with the same or lower bandgap as the emitterlayer but where the BSF layer may also be thought of as having the dualrole of a base layer since it forms a p-n junction with the emitter,such that the emitter layer is a major photoabsorbing region of thesolar cell, such that 50-100%, and preferably 90-100%, of thephotogenerated current density in the solar cell comes from the emitterlayer. The one or more LBAR and/or IT layers may be partly in the solarcell quasi-neutral region, and partly in the solar cell space-chargeregion.

In another aspect, a solar cell comprises emitter layers having bandgapslower than the base, BSF, and window regions, having improved chargecarrier transport properties in the emitter and/or forming one or morelow-bandgap absorber regions (1st level LBARs) in the emitter, andfurther having a lower bandgap absorber region (2nd level LBAR) withlower bandgap than the 1st level LBARs. The 2nd level LBAR may be partlyin the space-charge region and partly in the quasi-neutral region of thesolar cell, entirely within the space-charge region, or entirely withinthe quasi-neutral region.

In another aspect, the one or more LBAR and/or IT layers are positionedin the solar cell base in one of the following configurations: partly inthe base quasi-neutral region and partly in the base space-chargeregion; entirely in the base quasi-neutral region; or entirely in thebase space-charge region. The LBAR thickness may range from 0% to 100%of the total base thickness, and may be varied to affect current balancein a multijunction cell.

In another aspect, the one or more LBAR and/or IT layers are positionedpartly in the base quasi-neutral region and partly in the basespace-charge region, are positioned such that one or more of the LBARand/or IT layers is in contact with the emitter layer of a given dopingtype (n-type, p-type or intrinsic), and are positioned such that the oneor more LBAR and/or IT layers are separated from the back surface field(BSF) layer by an additional part of the base with doping type differentfrom that of the emitter. For example, in some implementations, the LBARlayer or the IT layer forms a first portion of a base layer of the celland the cell further comprises a window layer; a back-surface-field(BSF) layer; an emitter layer; and a second portion of the base layer.The first portion of the base layer is disposed between the emitterlayer and the second portion of the base layer. The second portion ofthe base layer is adjacent the BSF layer, and the first portion of thebase layer is positioned partially in the space-charge region of thebase layer and partially in the quasi-neutral region of the base layer.

In another aspect, the one or more LBAR and/or IT layers are positionedpartly in the base quasi-neutral region and partly in the basespace-charge region, are separated from the emitter layer by a firstadditional part of the base, and are separated from the back surfacefield (BSF) layer by a second additional part of the base. For example,in some implementations, the LBAR layer or the IT layer forms a firstportion of a base layer of the cell and the cell further comprises awindow layer; a back-surface-field (BSF) layer; an emitter layer; asecond portion of the base layer; and a third portion of the base layer.The first portion of the base layer is disposed between the second andthird portions of the base layer. The first portion of the base layer ispositioned partially in the space-charge region of the base layer andpartially in the quasi-neutral region of the base layer.

In another aspect, the one or more LBAR and/or IT layers are positionedentirely in the base quasi-neutral region, are separated from theemitter layer by an additional part of the base, and are positioned suchthat at least one LBAR and/or IT layer is in contact with the backsurface field (BSF) layer. For example, in some implementations, theLBAR layer or the IT layer forms a first portion of a base layer of thecell and the cell further comprises a window layer; a back-surface-field(BSF) layer; an emitter layer; and a second portion of the base layer.The first portion of the base layer is disposed between the BSF layerand the second portion of the base layer. The second portion of the baselayer is adjacent the emitter layer, and the first portion of the baselayer is positioned entirely in the quasi-neutral region of the baselayer.

In another aspect, the one or more LBAR and/or IT layers are positionedentirely in the base quasi-neutral region, are separated from theemitter layer by a first additional part of the base, and are separatedfrom the back surface field (BSF) layer by a second additional part ofthe base.

In another aspect, the one or more LBAR and/or IT layers form the entirebase region in a solar cell having an emitter with higher bandgap and/orreduced thickness with respect to the base, such that the base layer isa major photoabsorbing region of the solar cell, such that 30-100%, andpreferably 50-100%, of the photogeneration in the solar cell comes fromthe base layer. The base comprising one or more LBAR and/or IT layersmay be partly in the solar cell quasi-neutral region, and partly in thesolar cell space-charge region. The LBAR thickness may range from 5% to100% of the total solar cell photoabsorber thickness, and may be variedto affect current balance in a multijunction cell.

In another aspect, the one or more LBAR and/or IT layers form the entirebase region of a solar cell, comprising a p-n junction between the baselayer and a window layer with higher bandgap than the base layer, andfor which there is no emitter layer (zero thickness emitter layer) withthe same or lower bandgap as the base layer but where the window layermay also be thought of as having the dual role of an emitter layer sinceit forms a p-n junction with the base, such that the base layer is amajor photoabsorbing region of the solar cell, such that 50-100%, andpreferably 90-100%, of the photogenerated current density in the solarcell comes from the base layer. The one or more LBAR and/or IT layersmay be partly in the solar cell quasi-neutral region, and partly in thesolar cell space-charge region.

In another aspect, a solar cell comprises base layers having bandgapslower than the emitter, window, and BSF regions, having improved chargecarrier transport properties in the base and/or forming one or morelow-bandgap absorber regions (1st level LBARs) in the base, and furtherhaving a lower bandgap absorber region (2nd level LBAR) with lowerbandgap than the 1st level LBARs. The 2nd level LBAR may be partly inthe space-charge region and partly in the quasi-neutral region of thesolar cell, entirely within the space-charge region, or entirely withinthe quasi-neutral region.

In another aspect, the LBAR(s) and/or IT layer(s) comprise one or morezero-aluminum-content (aluminum-free) or low-aluminum-contentsemiconductor layers, where the primary photogeneration regions of thesolar cell have a finite, non-zero aluminum content, and wherelow-aluminum-content is defined as having lower aluminum compositionthan that of the primary photogeneration regions or adjacentphotogeneration regions of the solar cell, for example, the solar cellbase or emitter. For example, in some implementations, the LBAR layer orthe IT layer is free or substantially free of aluminum and at least onesemiconductor layer immediately adjacent the LBAR layer or the IT layerincludes aluminum and has a higher bandgap than the LBAR layer or the ITlayer. In some implementations, the LBAR layer or the IT layer comprisesno more than about 15 mole percent aluminum relative to the total amountof group III elements present in the layer and at least onesemiconductor layer immediately adjacent the LBAR layer or the IT layerincludes a higher mole percent of aluminum than the LBAR layer or the ITlayer and has a higher bandgap than the LBAR layer or the IT layer.Further, in some implementations, the cell of a device described hereinfurther comprises an emitter layer and a base layer, and the LBAR layeror the IT layer comprises no more than about 15 mole percent aluminumrelative to the total amount of group III elements present in the layer,and is positioned in or composes the entirety of the emitter layer. Inaddition, the base layer includes a higher mole percent of aluminum thanthe LBAR layer or the IT layer and has a higher bandgap than the LBARlayer or the IT layer.

Moreover, in some implementations, the LBAR layer or the IT layer of acell described herein forms a first portion of a base layer of the celland the cell further comprises a window layer; a back-surface-field(BSF) layer; an emitter layer; a second portion of the base layer; and athird portion of the base layer. The first portion of the base layer isdisposed between the second portion of the base layer and the thirdportion of the base layer. The second portion of the base layer isadjacent the emitter layer. The third portion of the base layer isdisposed between the first portion of the base layer and the BSF layer.The first portion of the base layer is positioned entirely in thequasi-neutral region of the base layer, entirely in the space-chargeregion, or partly in the quasi-neutral region and partly in thespace-charge region. Additionally, the first portion of the base layerhas a lower aluminum mole percent and lower bandgap than the second andthird portions of the base layer.

For reference purposes herein, a layer that is aluminum-free comprisesno aluminum or no intentionally added aluminum. A layer that issubstantially free of aluminum can comprise no more than about 1 atompercent aluminum, no more than about 0.1 atom percent aluminum, or nomore than about 0.01 atom percent aluminum, based on the total amount ofGroup III elements.

In another aspect, the zero- or low-aluminum-content LBAR(s) and/or ITlayer(s) form part or all of the solar cell emitter, where the base ofthe solar cell has a finite, non-zero aluminum content.

In another aspect, the LBAR and/or IT layers comprise azero-aluminum-content (Al-free) GaInP layer forming the entire emitterof a solar cell with an AlGaInP base, where the improved charge carriertransport properties and lower bandgap of the emitter compared to thebase are due to the absence of aluminum (Al) in the emitter composition.The GaInP emitter and the AlGaInP base may be ordered, disordered, orhave varying degrees of ordering on the group III sublattice.

In another aspect, the LBAR and/or IT layers comprise alow-aluminum-content AlGaInP layer forming the entire emitter of a solarcell with an AlGaInP base, where the AlGaInP emitter layer has lowaluminum content with respect to the Al composition of the base, andwhere the improved charge carrier transport properties and lower bandgapcompared of the emitter to the base are due to the lower content ofaluminum (Al) in the emitter composition. The low-Al-content AlGaInPemitter and the AlGaInP base may be ordered, disordered, or have varyingdegrees of atomic ordering on the group-III sublattice.

In another aspect, the zero- or low-aluminum-content LBAR(s) and/or ITlayer(s) form part or all of the solar cell base, where the emitter ofthe solar cell has a finite, non-zero aluminum content.

In another aspect, any of the semiconductor device structures in thispatent disclosure using an Al-free GaInP may instead have alow-Al-content AlGaInP IT and/or LBAR layer, with low Al compositionwith respect to other photogeneration layers in the solar cell.

In another aspect, the LBAR(s) and/or IT layers are formed from regionswith a higher or greater degree of group-III sublattice ordering, e.g.,ordering of Ga and In in GaInP, or ordering of Al, Ga, and In inAlGaInP, and lower bandgap with respect to adjacent regions with highergroup-III sublattice disorder and higher band gap. For referencepurposes herein, a layer having a higher or greater degree of group-IIIsublattice ordering exhibits a lower bandgap at the same latticeconstant compared to a layer having a lower degree of group-IIIsublattice ordering. In addition, a layer having a higher or greaterdegree of group-III sublattice ordering exhibits a higher orderparameter than exhibited by a layer having a lower degree of group-IIIsublattice ordering. For example, a layer having an order parameter ofless than about 0.2 can be considered to have a relatively low degree ofgroup-III sublattice ordering and can be referred to as beingdisordered. A layer having an order parameter of about 0.2 or more canbe considered to have a higher degree of group-III sublattice orderingand may be referred to as being ordered. Intermediate values of theorder parameter are also possible.

In another aspect, the LBAR(s) and/or IT layers are formed from regionswith a different degree of group-V sublattice ordering, e.g., orderingof As and Sb in GaAsSb, with respect to adjacent regions or layers.

In another aspect, the LBAR(s) and/or IT layers are formed from regionswith a different degree of anion or cation sublattice ordering in asemiconductor family, e.g., ordering in III-V, II-VI, I-III-VI,III-IV-VI semiconductors, with respect to adjacent regions.

In another aspect, a solar cell comprises an AlGaInP base, an Al-freeGaInP emitter forming an LBAR and improved transport (IT) layer in theemitter, in combination with a high-Al-content, pseudomorphic, AlInPwindow in tensile strain with respect to the emitter to achieve higherbandgap and greater transparency of the window. Here high-Al-content inthe AlInP emitter means that the Al content is higher than the AlInPcomposition with the same material lattice constant as the AlGaInP base,and the GaInP emitter is lattice-matched and unstrained with respect tothe AlGaInP solar cell base.

In another aspect, the LBAR(s) and/or IT layers are formed from regionswith higher indium content, and in compressive strain, e.g., higher-InGaInP, AlGaInP, GaInAs, AlGaInAs, GaInPAs, or AlGaInPAs regions orlayer, with respect to adjacent regions or layers with lower or zeroindium content, e.g., GaAs, AlGaAs, GaPAs, or lower-In GaInP, AlGaInP,GaInAs, AlGaInAs, GaInPAs, or AlGaInPAs regions.

In another aspect, the LBAR(s) or IT layers are formed in astrain-balanced structure comprising alternating layers ofcompressively-strained (compressive) and tensile-strained (tensile)semiconductor layers.

In another aspect, the compressive layers in a strain-balanced structureare LBARs with lower bandgap than the adjacent tensile layers, e.g.,GaInAs LBARs strain-balanced with GaPAs tensile (barrier) layers.

In another aspect, a solar cell comprises an AlGaInP base, with apseudomorphic, Al-free GaInP emitter in compressive strain with respectto the AlGaInP base, forming an LBAR and improved transport (IT) layerin the emitter, in combination with a high-Al-content, pseudomorphic,AlInP window in tensile strain with respect to the emitter to achievehigher bandgap and greater transparency of the window. Herehigh-Al-content in the AlInP emitter means that the Al content is higherthan the AlInP composition with the same material lattice constant asthe AlGaInP base, and the compressively-strained GaInP emitter balancesthe tensile strain in the AlInP window.

In another aspect, a solar cell comprises an AlGaInP base, with apseudomorphic, AlGaInP emitter in compressive strain with respect to theAlGaInP base and with Al content in the emitter such that the emitterbandgap may be less than, the same as, or greater than the AlGaInP base,in combination with a high-Al-content, pseudomorphic, AlInP window intensile strain with respect to the emitter to achieve higher bandgap andgreater transparency of the window. Here high-Al-content in the AlInPemitter means that the Al content is higher than the AlInP compositionwith the same material lattice constant as the AlGaInP base, and thecompressively-strained AlGaInP emitter balances the tensile strain inthe AlInP window.

In another aspect, a solar cell comprises a GaInP or AlGaInP base, withLBARs and strain compensation regions (SCRs) which may take the form oflayers. The LBARs and strain compensation regions may be partly in thespace-charge region and partly in the quasi-neutral region of the base,or may be partly in the space-charge region and partly in thequasi-neutral region of the emitter, or may be partly in thespace-charge region and partly in the quasi-neutral regions of both theemitter and the base, or may be entirely in the quasi-neutral region ofthe base, or may be entirely in the quasi-neutral region of the emitter,or may be entirely in the space-charge region. Any of the examplesdescribed herein with an improved transport (IT) and/or LBAR layer inthe cell structure may instead use a combination of LBARs and straincompensation regions.

In another aspect, the LBAR(s) and/or IT layers are formed from regionswith higher arsenic or antimony content, and in compressive strain,e.g., higher-As GaInPAs or higher-Sb GaInAsSb regions, with respect toadjacent regions with lower or zero arsenic or antimony content.

In another aspect, the improved transport (IT) layers have a bandgap orbandgaps that are the same as or higher than other layers in the solarcell, such as layers in or the entirety of the solar cell base oremitter. These other layers in the solar cell, other than the IT layers,may form the main photogeneration layers in the solar cell, or the ITlayers themselves may form the main photogeneration layers in the solarcell. These other layers in the solar cell, other than the IT layers,may be adjacent to the IT layers or separated from the IT layers.

In another aspect, the improved transport (IT) layers are formed from ahigh-Ga GaInP first layer in the emitter or window layer of a solarcell, in tensile strain with respect to other parts of the emitter orwindow and/or to other layers in the solar cell such as the solar cellbase, which is strain-balanced by a high-In AlGaInP or AlInP secondlayer, in the emitter, window, or base layers of the solar cell,adjacent to and in compressive strain with respect to the first high-GaGaInP first layer in the emitter or window. The Al content in theAlGaInP or AlInP second layer may be such that its bandgap is the sameas, lower than, or higher than the GaInP first layer. In the lattercase, the GaInP first layer may form an LBAR with respect to theadjacent layers. The combined high-Ga GaInP first layer and high-InAlGaInP or AlInP second layer may both form an IT layer together, andmay have compositions such that the bandgaps of both layers are the sameas, higher than, or lower than surrounding solar cell structures such aslayers within or the entirety of the solar cell base and/or windowlayers. In the former cases, where the bandgaps of both layers in thestrain-balanced IT layer may be chosen to be the same as or higher thanthat of the base layer, the relatively high bandgap of thestrain-balanced IT layer increases the voltage of the solar cell withrespect to the case with lower bandgap IT layers. In the latter case,where the bandgaps of both layers in the strain-balanced IT layer arelower than that of the base and/or window layers, the combined IT layermay form an LBAR with respect to the surrounding solar cell layers suchas the solar cell base, increasing the current density of the solarcell. In this context, high-Ga in the GaInP first layer refers to thatlayer having a Ga composition that results in a lattice constant that issmaller than that of the base layer, smaller than the other mainphotogeneration layers in the solar cell, and/or smaller than theaverage lattice constant of the layers in the solar cell. In thiscontext, high-In in the AlGaInP or AlInP second layer refers to thatlayer having an In composition that results in a lattice constant thatis larger than that of the base layer, larger than the other mainphotogeneration layers in the solar cell, and/or larger than the averagelattice constant of the layers in the solar cell.

In a particular implementation, the high-Ga GaInP first layer andhigh-In AlGaInP or AlInP second layer may have compositions and strainlevels such that the bandgaps of the first layer and the second layerare the same or approximately the same, and that these bandgaps are thesame, approximately the same, or higher than the bandgap of the layer orlayers forming the bulk of the solar cell base. In this implementation,the base may be chosen to have an AlGaInP base layer, the high-Ga GaInPfirst layer may be chosen to be in the emitter and to have a Gacomposition and tensile strain state such that its bandgap is the sameor approximately the same as the AlGaInP base layer, and the high-InAlGaInP or AlInP second layer may be chosen to be in the emitter and tohave an In composition, Al composition, and compressive strain statesuch that its bandgap is the same or approximately the same as the GaInPfirst layer, and as the AlGaInP base layer. In this context,approximately the same refers to the GaInP first layer and the AlGaInPor AlInP second layer having bandgaps that are less than 0.050 eVdifferent from each other, and/or to the first layer or second layerhaving a bandgap that is less than 0.050 eV from the AlGaInP base.

In another aspect, the strain-balanced high-Ga GaInP/high-In AlGaInP ITlayer system described herein may be implemented using other elements,material systems, and semiconductor families, including but not limitedto the high-Ga GaInAs/high-In AlGaInAs; high-Ga GaInAs/high-In GaInPAs,high-Ga GaAsSb/high-In or high-Sb AlGaInSb, high-Ga GaInAs/high-In orhigh-Sb GaInNAsSb, high-Si SiGe/low-Si SiGe, AlGaInPAsSb/AlGaInPAsSb,GaInNAsSb/GaInNAsSb, and SiGeSn/SiGeSn material systems.

In another aspect, the LBAR(s) or IT layers are formed from regions withnon-zero, dilute nitrogen content, e.g., GaNAs, GaNAs(Sb), GaInNAs, orGaInNAsSb regions with nitrogen content in the range of 0.01% to 10%,with reduced bandgap due to the incorporation of nitrogen in thesemiconductor alloy. For example, in some implementations, an LBAR layeror IT layer of a cell described herein includes nitrogen and at leastone semiconductor layer immediately adjacent the LBAR layer or the ITlayer is free or substantially free of nitrogen and has a higher bandgapthan the LBAR layer or the IT layer. In some implemenations, an LBARlayer or IT layer includes nitrogen and at least one semiconductor layerimmediately adjacent the LBAR layer or the IT layer includes a non-zeroamount of nitrogen, the immediately adjacent semiconductor layer havinga higher bandgap than the LBAR layer or the IT layer and a lower molepercent of nitrogen than the LBAR layer or the IT layer. Moreover, insome implementations, a cell described herein further comprises anemitter layer and a base layer having a higher bandgap than the emitterlayer, wherein the LBAR layer including nitrogen is disposed in theemitter layer, and the base layer has a lower nitrogen content than theLBAR layer and forms an IT layer. In other instances, a cell describedherein comprises a base layer and an emitter layer having a higherbandgap than the base layer wherein an LBAR layer including nitrogen isdisposed in the base layer and an emitter layer having a higher bandgapand a lower nitrogen content than the LBAR layer forms an IT layer.

In another aspect, a solar cell comprises a dilute nitride GaInNAs(Sb)emitter layer forming an LBAR in the emitter due to the reduction inbandgap due to nitrogen incorporation, and a GaAs, GaInAs, orlow-nitrogen-content GaInNAs(Sb) base, with lower N content and higherbandgap with respect to the emitter, forming an improved transport (IT)layer in the base, due to the absence or lower concentration of N in thebase layers.

In another aspect, a solar cell comprises a dilute nitride GaInNAs(Sb)base layer forming an LBAR in the base due to the reduction in bandgapdue to nitrogen incorporation, and a GaAs, GaInAs, orlow-nitrogen-content GaInNAs(Sb) emitter, with lower N content andhigher bandgap with respect to the base, forming an improved transport(IT) layer in the emitter, due to the absence or lower concentration ofN in the emitter layers.

In another aspect, a solar cell comprises a thick dilute nitrideGaInNAs(Sb) emitter layer, forming an LBAR due to the reduction inbandgap due to nitrogen incorporation, such that the emitter layer is amajor photoabsorbing region of the solar cell, such that 30-100%, andpreferably 50-100%, of the photogeneration in the solar cell comes fromthe emitter layer, and with an optional GaAs, GaInAs, orlow-nitrogen-content GaInNAs(Sb) base, with lower N content and higherbandgap with respect to the emitter.

In another aspect, a solar cell comprises a dilute nitride GaInNAs(Sb)layer within the base, forming an LBAR in the base due to the reductionin bandgap due to nitrogen incorporation, and with additional baselayers and emitter layers composed of GaAs, GaInAs, orlow-nitrogen-content GaInNAs(Sb), having lower N content and higherbandgap with respect to the base LBAR layer, forming improved transport(IT) layers in the base and emitter, due to the absence or lowerconcentration of N.

In another aspect, the LBAR(s) or IT layers are formed from regions withhigher nitrogen content, and in tensile strain, e.g., higher-N GaNAs,GaNAs(Sb), GaInNAs, or GaInNAsSb regions with nitrogen content in therange of 0.01% to 10%, with respect to adjacent regions with lower orzero nitrogen content.

In another aspect, the LBAR(s) or IT layers are formed from Ge, SiGe, orSiGeSn regions with lower silicon content with respect to adjacent SiGeor SiGeSn regions.

In another aspect, the tensile layers in a strain-balanced structure areLBARs with lower bandgap than the compressive layers in between, e.g.,GaNAs LBARs strain-balanced with higher bandgap compressive layers inbetween.

In another aspect, both the compressive and tensile layers in astrain-balanced structure are both lower in bandgap than regionssurrounding the strain-balanced structure, and thus both compressive andtensile layers contribute to the thickness of an LBAR with the thicknessof the overall strain-balanced structure, e.g., GaInAs(Sb) low-bandgapcompressive layers strain-balanced with GaNAs(Sb) low-bandgap tensilelayers in between, forming a wide LBAR with the thickness equal to thesum of the GaInAs(Sb) and GaNAs(Sb) layer thicknesses, with bandgaplower than that of the GaAs or GaInAs host material (e.g., layers in thesolar cell base or emitter) adjacent to the strain-balanced structure.

In another aspect, a solar cell comprises alternating GaNAs(Sb) layersin the base in tensile strain, and GaInAs(Sb) layers in the base incompressive strain balancing the strain in the tensile-strain layers,such that both tensile and compressive layers have lower bandgap than aGaAs or low-indium-content GaInAs base layer, forming LBARs in both thetensile GaNAs(Sb) and compressive GaInAs(Sb) layers in the base, andadditionally forming improved transport (IT) layers in the compressiveGaInAs(Sb) layers, due partly to the absence of nitrogen.

In another aspect, the IT and/or LBAR layers may be semiconductor layersin a solar cell emitter and/or window layer, with elimination,reduction, addition, or increase of a single element with respect to thesemiconductor composition of the main photogeneration layer or layers inthe solar cell such as layers in the solar cell base or emitter, forbinary (n=2), ternary (n=3), quaternary (n=4), pentanary (n=5)semiconductor compositions or semiconductor compositions with n=6 orgreater in the main photogeneration layers, where n is the number ofdifferent elements in the semiconductor composition of the mainphotogeneration layer or layers.

In another aspect, the IT and/or LBAR layers may be semiconductor layersin a solar cell emitter and/or window layer, with elimination,reduction, addition, or increase of two or more elements with respect tothe semiconductor composition of the main photogeneration layer orlayers in the solar cell such as layers in the solar cell base oremitter, for binary (n=2), ternary (n=3), quaternary (n=4), pentanary(n=5) semiconductor compositions or semiconductor compositions with n=6or greater in the main photogeneration layers, where n is the number ofdifferent elements in the semiconductor composition of the mainphotogeneration layer or layers.

In another aspect, the IT and/or LBAR layers may consist of an emitteror window with no nitrogen or reduced nitrogen content relative to oneor more nitrogen-containing layers in the base, intrinsic region, orspace-charge region of a solar cell that includes nitrogen in itssemiconductor composition. In a particular implementation, thenitrogen-containing layer or layers may include a main photogenerationlayer or layers of the solar cell. In a particular implementation, thebase region, intrinsic region, or space-charge region layer may becomposed of GaInNAs or GaInNAsSb, and the emitter and/or window layermay be composed of GaAs, GaInAs, GaAsSb, or GaInAsSb.

In another aspect, the IT and/or LBAR layers may consist of a base,back-surface-field (BSF) layer, or back heterojunction layer with nonitrogen or reduced nitrogen content relative to one or morenitrogen-containing layers in the window, emitter, intrinsic region, orspace-charge region of a solar cell that includes nitrogen in itssemiconductor composition. In a particular implementation, thenitrogen-containing layer or layers may include a main photogenerationlayer or layers of the solar cell. In a particular implementation, theemitter region, intrinsic region, or space-charge region layer may becomposed of GaInNAs or GaInNAsSb, and the base, back-surface-field (BSF)layer, and/or back heterojunction layer may be composed of GaAs, GaInAs,GaAsSb, or GaInAsSb.

In another aspect, the IT and/or LBAR layers may consist of an intrinsicregion or space-charge region layer with no nitrogen or reduced nitrogencontent relative to one or more nitrogen-containing layers in thewindow, emitter, base, back-surface field (BSF) layer, or backheterojunction layer of a solar cell that includes nitrogen in itssemiconductor composition. In a particular implementation, thenitrogen-containing layer or layers may include a main photogenerationlayer or layers of the solar cell. In a particular implementation, theintrinsic region, or space-charge region layer may be composed ofGaInNAs or GaInNAsSb, and the window, emitter, base, back-surface-field(BSF) layer, and/or back heterojunction layer may be composed of GaAs,GaInAs, GaAsSb, or GaInAsSb.

In a preferred implementation, the nitrogen composition of thenitrogen-containing layer or layers may be chosen to be between 0.1% and10% nitrogen, and more preferably between 0.2% and 5% nitrogen, andstill more preferably between 0.5% and 3% nitrogen.

In still another implementation, the nitrogen composition of thenitrogen-containing layer or layers may be chosen to be between 45% and50% nitrogen, and more preferably between 49% and 50% nitrogen, andstill more preferably 50% nitrogen such that nitrogen forms the entiretyof the group-V elemental component of the semiconductor, for example, asin GaN, AlN, InN, AlGaN, GaInN, AlInN, and AlGaInN semiconductors.

In another aspect, a solar cell comprises an AlGa(In)As base, with anAl-free Ga(In)As or low-Al-content AlGa(In)As layer in the emitterand/or base, forming an LBAR and/or improved transport (IT) layer, wherethe improved charge carrier transport properties may result from theabsence or low concentration of aluminum (Al) in the LBAR and/or ITlayer. The LBAR and/or IT layer may be partly in the space-charge regionand partly in the quasi-neutral region of the base, or may be partly inthe space-charge region and partly in the quasi-neutral region of theemitter, or may be partly in the space-charge region and partly in thequasi-neutral regions of both the emitter and the base, or may beentirely in the quasi-neutral region of the base, or may be entirely inthe quasi-neutral region of the emitter, or may be entirely in thespace-charge region. The LBAR and/or IT layer thickness may be varied toaffect current balance in the multijunction cell, for example, thethickness may range from 0% to 100% of the total emitter thickness. Inaddition, the emitter thickness may vary from 0% (no emitter case) to100% (all emitter case) of the combined emitter thickness plus the basethickness. In general, the LBAR and/or IT layer or layers may have anyof the configurations described herein.

In another aspect, a solar cell comprises a Ga(In)PAs base, with anP-free Ga(In)As or low-P-content Ga(In)PAs layer within the emitterand/or base, forming an LBAR and/or improved transport (IT) layer, wherethe improved charge carrier transport properties may result from theabsence or low concentration of phosphorus (P) in the LBAR and/or ITlayer. The LBAR and/or IT layer may be partly in the space-charge regionand partly in the quasi-neutral region of the base, or may be partly inthe space-charge region and partly in the quasi-neutral region of theemitter, or may be partly in the space-charge region and partly in thequasi-neutral regions of both the emitter and the base, or may beentirely in the quasi-neutral region of the base, or may be entirely inthe quasi-neutral region of the emitter, or may be entirely in thespace-charge region. The LBAR and/or IT layer thickness may be varied toaffect current balance in the multijunction cell, for example, thethickness may range from 0% to 100% of the total emitter thickness. Inaddition, the emitter thickness may vary from 0% (no emitter case) to100% (all emitter case) of the combined emitter thickness plus the basethickness. In general, the LBAR and/or IT layer or layers may have anyof the configurations described herein.

In another aspect, a solar cell comprises a Ga(In)(N)(P)As base, andwith an Sb-containing Ga(In)(N)(P)AsSb layer within the emitter and/orbase, forming an LBAR and/or improved transport (IT) layer, where theimproved charge carrier transport properties may result from thepresence of antimony (Sb) in the LBAR and/or IT layer, or during thegrowth of the LBAR and/or IT layer. The LBAR and/or IT layer may bepartly in the space-charge region and partly in the quasi-neutral regionof the base, or may be partly in the space-charge region and partly inthe quasi-neutral region of the emitter, or may be partly in thespace-charge region and partly in the quasi-neutral regions of both theemitter and the base, or may be entirely in the quasi-neutral region ofthe base, or may be entirely in the quasi-neutral region of the emitter,or may be entirely in the space-charge region. The LBAR and/or IT layerthickness may be varied to affect current balance in the multijunctioncell, for example, the thickness may range from 0% to 100% of the totalemitter thickness. In addition, the emitter thickness may vary from 0%(no emitter case) to 100% (all emitter case) of the combined emitterthickness plus the base thickness. In general, the LBAR and/or IT layeror layers may have any of the configurations described herein.

In another aspect, the improved transport (IT) layers described hereinmay be positioned partly or entirely within the space charge region of asolar cell. In this configuration the improved charge-carrier mobility,charge-carrier recombination lifetime, and/or charge-carrier saturationvelocity in an electric field resulting from the chemical composition,degree of sublattice ordering, strain state, low concentration ofcrystal defects present, and/or type of crystal defects present in theIT layer enhances carrier transport in the solar cell space chargeregion, increasing the solar cell current density, voltage, and/or fillfactor, resulting in higher solar cell efficiency.

In another aspect, the material bandgap of the one or more LBARs is from0 to 50 milli-electron volts (meV) less than that of the bulk ofphotogeneration or light-absorbing material in a solar cell or otheroptoelectronic device, or of the one or more layers adjacent to the oneor more LBARs.

In another aspect, the material bandgap of the one or more LBARs is from50 to 150 meV less than that of the bulk of photogeneration orlight-absorbing material in a solar cell or other optoelectronic device,or of the one or more layers adjacent to the one or more LBARs.

In another aspect, the material bandgap of the one or more LBARs is from150 to 300 meV less than that of the bulk of photogeneration or lightabsorbing material in a solar cell or other optoelectronic device, or ofthe one or more layers adjacent to the one or more LBARs.

In another aspect, the material bandgap of the one or more LBARs is morethan 300 meV lower than that of the bulk of photogeneration orlight-absorbing material in a solar cell or other optoelectronic device,or of the one or more layers adjacent to the one or more LBARs.

In another aspect, the LBAR or improved transport (IT) layers describedherein may be combined in a solar cell with a reflector structureincorporated into the solar cell structure, where the reflectorstructure reflects light back into the one or more LBAR, IT, or othersubcell layers, such that the light has a non-zero probability of beingabsorbed and producing useful current in the solar cell, and/orincreasing solar cell voltage after being reflected back into the LBAR,IT, or other subcell layers, and where the reflector structure is chosenfrom a list of reflector types including:

-   -   a semiconductor layer with different refractive index than one        or more semiconductor layers in the solar cell forming a        semiconductor layer reflector;    -   a stack of semiconductor layers with different refractive        indices forming a semiconductor Bragg reflector;    -   a dielectric layer with different refractive index than one or        more semiconductor layers in the solar cell forming a dielectric        layer reflector;    -   a stack of dielectric layers with different refractive indices        forming a dielectric Bragg reflector;    -   a metal layer, patterned metal regions, or unpatterned        conductive particles, nanowires, or regions;    -   a composite semiconductor/metal layer reflector structure;    -   a composite semiconductor/patterned metal region reflector        structure, where the metal regions may be imbedded in the        semiconductor material;    -   a composite semiconductor/unpatterned conductive particle,        nanowire, or region reflector structure, where the conductive        particles, nanowires, or regions may be imbedded in the        semiconductor material;    -   a composite dielectric/metal layer reflector structure;    -   a composite dielectric/patterned metal region reflector        structure, where the metal regions may be imbedded in the        dielectric material;    -   a composite dielectric/unpatterned conductive particle,        nanowire, or region reflector structure; and    -   combinations of the above components in a reflector structure.        The reflector structures may be designed such that they        efficiently transmit light with photon energy that can be used        effectively by subcells in a multijunction solar cell structure        positioned beneath the reflector structure, or with photon        energy that cannot be used effectively by the LBAR or IT layers,        or other parts of the subcell, above the reflector structure, or        both.

In another aspect, a reflector structure described herein combined withone or more LBAR or IT layers reflects light incident on the solar cellthat is not fully absorbed by the LBAR or IT layers in its initial passthrough the layers, but that has a photon energy that is either greaterthan the lowest bandgap of the LBAR or IT layers, or is no more than 4kT less than the lowest bandgap of the LBAR or IT layers (where k is theBoltzmann constant and T is the operating temperature of the solar cellin Kelvin), such that the light has a non-zero probability of beingabsorbed and producing useful current in the solar cell, and/orincreasing solar cell voltage after the light is reflected by thereflector structure back into the LBAR or IT layers for a second pass orfor multiple passes through the LBAR or IT layers.

In another aspect, a reflector structure described herein combined withone or more LBAR or IT layers reflects light emitted by radiativerecombination of electrons and holes in the one or more LBAR or ITlayers, such that the emitted photons are reflected back into the one ormore LBAR or IT layers and may be reabsorbed in the LBAR or IT layers toproduce electron-hole pairs through the physical mechanism ofelectron-hole pair photogeneration which is the inverse mechanism ofradiative recombination, increasing photon recycling in the LBAR or ITlayers, and thereby decreasing the net rate of electron-holerecombination, increasing the effective minority-carrier recombinationlifetime, increasing the splitting of electron and hole quasi-Fermilevels, and increasing the voltage and efficiency of the solar cell.

In another aspect, the LBAR or IT layers described herein may comprise asemiconductor material selected from a list of preferred materialsincluding: InP, GaInP, AlInP, AlGaInP, GaAs, AlGaAs, GaInAs, AlInAs,AlGaInAs, GaPAs, InPAs, GaInPAs, AlInPAs, AlGaInPAs, GaNAs, GaInNAs,GaNAsSb, GaInNAsSb, GaSb, GaInSb, AlGaInSb, GaAsSb, GaInAsSb,AlGaInAsSb, Si, Ge, SiGe.

In another aspect, layers between the LBAR or IT layers describedherein, such as strain-compensation-region (SCR) layers, may comprise asemiconductor material selected from a list of preferred materialsincluding: InP, GaInP, AlInP, AlGaInP, GaAs, AlGaAs, GaInAs, AlInAs,AlGaInAs, GaPAs, InPAs, GaInPAs, AlInPAs, AlGaInPAs, GaNAs, GaInNAs,GaNAsSb, GaInNAsSb, GaSb, GaInSb, AlGaInSb, GaAsSb, GaInAsSb,AlGaInAsSb, Si, Ge, SiGe.

In another aspect, the LBAR or IT layers described herein may be used ina solar cell for which the base layer, emitter layer, and/or primaryphotogeneration layer (the layer in the solar cell for whichphotogenerated current density is greatest) comprises a semiconductormaterial selected from a list of preferred materials including: InP,GaInP, AlInP, AlGaInP, GaAs, AlGaAs, GaInAs, AlInAs, AlGaInAs, GaPAs,InPAs, GaInPAs, AlInPAs, AlGaInPAs, GaNAs, GaInNAs, GaNAsSb, GaInNAsSb,GaSb, GaInSb, AlGaInSb, GaAsSb, GaInAsSb, AlGaInAsSb, Si, Ge, SiGe.

In another aspect, the LBAR or IT layers described herein may comprise,layers between the LBAR or IT layers such as strain-compensation-region(SCR) layers may comprise, or the LBAR or IT layers described herein maybe used in a solar cell for which the base layer, emitter layer, and/orprimary photogeneration layer (the layer in the solar cell for whichphotogenerated current density is greatest) comprises, a semiconductormaterial selected from an expanded list of materials including: InP,GaInP, AlInP, AlGaInP, GaAs, InAs, AlGaAs, GaInAs, AlInAs, AlGaInAs,GaPAs, InPAs, AlGaPAs, GaInPAs, AlInPAs, AlGaInPAs, GaNAs, GaInNAs,GaNAsSb, GaInNAsSb, GaNP, GaInNP, GaN, AlGaN, GaInN, GaSb, GaInSb,AlGaInSb, GaAsSb, GaInAsSb, AlGaInAsSb, GaPAsSb, GaInPAsSb, AlGaInPAsSb,Si, Ge, SiGe, SiSn, GeSn, SiGeSn, CSiGeSn, ZnO, CdS, ZnSe, CdSe, ZnTe,CdTe, CuInS, CuGaInS, CuInSe, CuGaInSe, CuGaInSSe.

In another aspect, the LBAR or IT layers described herein may comprise,layers between the LBAR or IT layers such as strain-compensation-region(SCR) layers may comprise, or the LBAR or IT layers described herein maybe used in a solar cell for which the base layer, emitter layer, and/orprimary photogeneration layer (the layer in the solar cell for whichphotogenerated current density is greatest) comprises, a semiconductormaterial selected from a further expanded list of materials including:AlP, GaP, InP, AlGaP, GaInP, AlInP, AlGaInP, AlAs, GaAs, InAs, AlGaAs,GaInAs, AlInAs, AlGaInAs, AlPAs, GaPAs, InPAs, AlGaPAs, GaInPAs,AlInPAs, AlGaInPAs, AlNAs, GaNAs, InNAs, AlGaNAs, GaInNAs, AlInNAs,AlGaInNAs, AlNAsSb, GaNAsSb, InNAsSb, AlGaNAsSb, GaInNAsSb, AlInNAsSb,AlGaInNAsSb, AlNP, GaNP, InNP, AlGaNP, GaInNP, AlInNP, AlGaInNP,GaInNPAs, GaInNPAsSb, AlGaInNPAsSb, AlN, GaN, InN, AlGaN, GaInN, AlInN,AlGaInN, AlSb, GaSb, InSb, AlGaSb, GaInSb, AlInSb, AlGaInSb, AlAsSb,GaAsSb, InAsSb, AlGaAsSb, GaInAsSb, AlInAsSb, AlGaInAsSb, AlPAsSb,GaPAsSb, InPAsSb, AlGaPAsSb, GaInPAsSb, AlInPAsSb, AlGaInPAsSb, C, Si,Ge, SiGe, SiSn, GeSn, SiGeSn, CSi, CGe, CSn, CSiGe, CGeSn, CSiSn,CSiGeSn, ZnO, CdO, ZnS, CdS, ZnSe, CdSe, ZnTe, CdTe, CuGaS, CuInS,CuGaInS, CuGaSe, CuInSe, CuGaInSe, CuGaSSe, CuInSSe, CuGaInSSe.

In another aspect, a cell described herein incorporating one or moreLBAR or IT layers forms a subcell within a multijunction solar cell.

In another aspect, a cell described herein incorporating one or moreLBAR or IT layers forms a subcell within a multijunction solar cellhaving one or more tunnel junctions between subcells, and may optionallyhave graded buffer layers in inverted metamorphic (IMM) and/or uprightmetamorphic (UMM) cells, dielectric/metal bonding layers, semiconductorbonding layers between the subcells in the multijunction cell.

In another aspect, a cell described herein incorporating LBAR or ITlayers forms at least one subcell within a 2-junction solar cell,3-junction solar cell, 4-junction solar cell, 5-junction solar cell,6-junction solar cell, 7-junction solar cell, or 8-junction solar cell.In another aspect, a cell described herein incorporating LBAR or ITlayers forms at least one subcell within a multijunction solar cell with9 or more junctions.

In another aspect, a multijunction solar cell described hereinincorporating LBAR or IT layers comprises subcells with the preferredsubcell semiconductor compositions and/or bandgaps listed in FIGS.34-40.

In another aspect, a multijunction solar cell described herein comprisesa top subcell, or cell 1 (C1) having a base with a bandgap selectedwithin a preferred range 1.7 to 2.7 eV, or from a more preferred rangeof 1.85 to 2.3 eV, and further comprising a combined LBAR and IT layerthat forms the top subcell emitter with a bandgap selected within apreferred range 1.6 to 2.2 eV, or from a more preferred range of 1.75 to2.1 eV.

In another aspect, a cell described herein incorporating LBAR or ITlayers forms a subcell within a multijunction solar cell comprisingsubcells with one or more of the following types of solar cell structureand orientation:

-   -   an upright epitaxially-grown structure, with sunward layers        grown after layers away from sun and/or high bandgap subcells        grown after subcells with lower bandgap;    -   an upright lattice-matched (ULM) multijunction cell structure;    -   an upright metamorphic (UMM) multijunction cell structure;    -   an inverted epitaxially-grown structure, with sunward layers        grown before layers away from sun and/or high bandgap subcells        grown before subcells with lower bandgap;        -   an inverted lattice-matched (ILM) multijunction cell            structure;        -   an inverted metamorphic (IMM) multijunction cell structure;    -   a semiconductor-to-semiconductor bonded multijunction cell        structure;    -   a dielectric bonded multijunction cell structure;    -   a metal bonded multijunction cell structure;    -   an adhesive bonded multijunction cell structure; and    -   an spectral splitting system in which solar cells are optically        integrated.

In another aspect, a cell described herein incorporating LBAR or ITlayers forms a subcell within a multijunction solar cell with one ormore of the following types of solar cell interconnection structure:

-   -   upright epitaxially-grown tunnel junction structures, with        layers closer to the sun in normal solar cell operation grown        after layers farther from the sun;    -   inverted epitaxially-grown tunnel junction structures, with        layers closer to the sun in normal solar cell operation grown        before layers farther from the sun;    -   semiconductor-to-semiconductor bonded interface interconnection        structure;    -   dielectric-bonded interface interconnection structure;    -   metal-bonded interface interconnection structure;    -   adhesive-bonded interface interconnection structure;    -   transparent-conductive-coating bonded interface interconnection        structure;    -   hybrid dielectric/metal bonded interface interconnection        structure;    -   hybrid adhesive/metal bonded interface interconnection        structure;    -   hybrid dielectric/adhesive bonded interface interconnection        structure; and    -   optical integration between subcells, as in a spectral splitting        system.

In another aspect, other optoelectronic devices are described herein. Insome implementations, for example, an optoelectronic device comprises aphotovoltaic cell, the photovoltaic cell comprising a space-chargeregion, a quasi-neutral region, a LBAR layer or an IT layer positionedat least partially in the space-charge region, one or more semiconductorlayers of a first layer type having a first bandgap and a first amountof strain with respect to the average lattice constant of the cell, andone or more semiconductor layers of a second layer type having a secondbandgap that is greater than the first bandgap and a second amount ofstrain. The layers of the second layer type are in tensile strain withrespect to the layers of the first layer type and the strain of thelayers of the first layer type is balanced with the strain of the layersof the second layer type such that the layers of both layer types remainpseudomorphic and retain a coherent lattice structure with a crystaldefect areal density lower than about 10⁶ cm⁻². In otherimplementations, an optoelectronic device comprises a photovoltaic cellwith the structure described above, but with the LBAR layer or IT layerpositioned at least partially in a quasi-neutral region.

In yet another aspect, still other optoelectronic devices are describedherein. In some implementations, for example, an optoelectronic devicecomprises a photovoltaic cell, the photovoltaic cell comprising aspace-charge region; a quasi-neutral region; a low bandgap absorberregion (LBAR) layer or an improved transport (IT) layer; one or moresemiconductor layers of a first layer type having a first nitrogencontent, a first indium or antimony content, a first bandgap, and afirst strain value; and one or more semiconductor layers of a secondlayer type having a second nitrogen content, a second indium or antimonycontent, a second bandgap, and a second strain value. The secondnitrogen content is zero, substantially zero, or less than the firstnitrogen content. The second indium or antimony content is greater thanthe first indium or antimony content. The second bandgap is lower than areplacement layer having zero indium content and zero antimony content.The layers of the second layer type are in compressive strain withrespect to the layers of the first layer type, and the first and secondstrain values are strain balanced such that the first and second layertypes remain pseudomorphic and retain a coherent lattice structure witha crystal defect areal density lower than 10⁶ cm⁻².

In addition, in some implementations, the cell further comprises aphotoabsorber region composed of an emitter layer and a base layer, theLBAR layer is composed of one or more layers of a first layer typestrain-balanced with one or more layers of a second layer type, and thebandgap of the first layer type and the bandgap of the second layer typeare lower than the bandgap of at least one other layer in thephotoabsorber region. Moreover, in some implementations, the LBAR layeris composed of one or more layers of a first layer type strain-balancedwith one or more layers of a second layer type, and the LBAR layer formsall or substantially all of the photoabsorber region of the cell.

One advantage of the present disclosure, in some implementations, is toincrease photogeneration of charge carriers in solar cells and otheroptoelectronic devices. Another advantage of the present disclosure, insome implementations, is to increase the collected photogeneratedcurrent density of a solar cell in a given solar spectrum. Anotheradvantage, in some implementations, is to improve the minority-carriertransport properties of one or more layers in a solar cell, such asminority-carrier lifetime, mobility, and diffusion length, and toimprove the short wavelength response of a solar cell. Still anotheradvantage, in some implementations, is to improve the majority-carriertransport properties of one or more layers in a solar cell, such asmajority-carrier mobility and conductance. Another advantage, in someimplementations, is to allow the use of wider bandgap top subcells in amultijunction cell, without degrading the minority-carrier propertiesand short wavelength response associated with the top cell emitter.Another advantage of the present disclosure, in some implementations, isto increase the energy conversion efficiency of a multijunction solarcell. Another advantage, in some implementations, is to provide a methodfor tuning the effective bandgap of one or more subcells in amultijunction solar cell while maintaining a given lattice constant in asubcell. Another advantage, in some implementations, is to improve thematch of the subcell effective bandgap combination to that of the solarspectrum and improve the solar cell efficiency.

Other features and advantages of the present disclosure will be apparentfrom the following more detailed description implementation, taken inconjunction with the accompanying drawings, which illustrate, by way ofexample, the principles of the disclosure implementation

BRIEF DESCRIPTION OF THE DRAWINGS

The drawings referenced herein form a part of the specification.Features shown in the drawings are meant as illustrative of some, butnot all, implementations of the disclosure, unless otherwise explicitlyindicated, and implications to the contrary are otherwise not to bemade. Wherever possible, the same reference numbers will be usedthroughout the drawings to represent the same or similar, though notnecessarily identical, components and/or features. However, for the sakeof brevity, reference numerals or features having a previously describedfunction may not necessarily be described in connection with otherdrawings in which such components and/or features appear.

FIG. 1 illustrates a device according to one implementation describedherein.

FIG. 2 illustrates a device according to one implementation describedherein.

FIG. 3 illustrates a device according to one implementation describedherein.

FIG. 4 illustrates a device according to one implementation describedherein.

FIG. 5 illustrates a device according to one implementation describedherein.

FIG. 6 illustrates a device according to one implementation describedherein.

FIG. 7 illustrates a device according to one implementation describedherein.

FIG. 8 illustrates a device according to one implementation describedherein.

FIG. 9 illustrates a device according to one implementation describedherein.

FIG. 10 illustrates a device according to one implementation describedherein.

FIG. 11 illustrates a device according to one implementation describedherein.

FIG. 12 illustrates a device according to one implementation describedherein.

FIG. 13 illustrates a device according to one implementation describedherein.

FIG. 14 illustrates a device according to one implementation describedherein.

FIG. 15 illustrates a device according to one implementation describedherein.

FIG. 16 illustrates a device according to one implementation describedherein.

FIG. 17 illustrates a device according to one implementation describedherein.

FIG. 18 illustrates a device according to one implementation describedherein.

FIG. 19 illustrates a device according to one implementation describedherein.

FIG. 20 illustrates a device according to one implementation describedherein.

FIG. 21 illustrates a device according to one implementation describedherein.

FIG. 22 illustrates a device according to one implementation describedherein.

FIG. 23 illustrates a device according to one implementation describedherein.

FIG. 24 illustrates a device according to one implementation describedherein.

FIG. 25 illustrates a device according to one implementation describedherein.

FIG. 26 illustrates a device according to one implementation describedherein.

FIG. 27 illustrates a device according to one implementation describedherein.

FIG. 28 illustrates a device according to one implementation describedherein.

FIG. 29 illustrates a device according to one implementation describedherein.

FIG. 30 illustrates a device according to one implementation describedherein.

FIG. 31 illustrates a device according to one implementation describedherein.

FIG. 32 illustrates a device according to one implementation describedherein.

FIG. 33 illustrates a device according to one implementation describedherein.

FIG. 34 illustrates a device according to one implementation describedherein.

FIG. 35 illustrates a device according to one implementation describedherein.

FIG. 36 illustrates a device according to one implementation describedherein.

FIG. 37 illustrates a device according to one implementation describedherein.

FIG. 38 illustrates a device according to one implementation describedherein.

FIG. 39 illustrates a device according to one implementation describedherein.

FIG. 40 illustrates a device according to one implementation describedherein.

FIG. 41 illustrates a plot of the external quantum efficiency of acontrol device.

FIG. 42 illustrates a plot of the external quantum efficiency of adevice according to one implementation described herein.

FIG. 43 illustrates a plot of the external quantum efficiency ofdevices, including devices according to some implementations describedherein.

DETAILED DESCRIPTION

In the following detailed description of representative implementationsof the disclosure, reference is made to the accompanying drawings thatform a part hereof, and in which are shown by way of illustrationspecific examples of implementations in which the disclosure may bepracticed. While these implementations are described in sufficientdetail to enable those skilled in the art to practice the invention, itwill nevertheless be understood that no limitation of the scope of thepresent disclosure is thereby intended. Alterations and furthermodifications of the features illustrated herein, and additionalapplications of the principles illustrated herein, which would occur toone skilled in the relevant art and having possession of thisdisclosure, are to be considered within the scope of this disclosure.Specifically, other implementations may be utilized, and logical,mechanical, electrical, material, and other changes may be made withoutdeparting from the spirit or scope of the present disclosure.

Accordingly, the following detailed description is not to be taken in alimiting sense, and the scope of the present disclosure is defined bythe appended claims. In addition, all ranges disclosed herein are to beunderstood to encompass any and all subranges subsumed therein. Forexample, a stated range of “1.0 to 10.0” should be considered to includeany and all subranges beginning with a minimum value of 1.0 or more andending with a maximum value of 10.0 or less, e.g., 1.0 to 5.3, or 4.7 to10.0, or 3.6 to 7.9.

The present disclosure describes a semiconductor device structurehaving, in some implementations, increased photogenerated currentdensity and greater collected current density. The device structure mayalso have increased voltage, and/or increased fill factor of itscurrent-voltage (I-V) characteristics. The device structure incorporateslow bandgap absorber regions (LBARs) composed of semiconductor materialsto increase photogenerated current density. LBARs may have a lowerbandgap by virtue of having a different semiconductor composition thanthe surrounding material, having a different degree of group-IIIsublattice or group-V sublattice disordering than the surroundingmaterial for LBARs composed of III-V semiconductors, having a differentamount of strain with respect to the surrounding material, orcombinations of the above, in addition to other physical phenomena thatcan cause a lowering of energy bandgap. LBARs may be lattice-matched tothe surrounding semiconductor material, having the same material latticeconstant as the surrounding semiconductor material, or they may have amaterial lattice constant that is either larger or smaller than that ofthe surrounding semiconductor material. Here the material latticeconstant is the lattice constant of a material in its unstrained state.LBARs may be unstrained or strained with respect to the surroundingsemiconductor material, and any strain in the LBARs may bestrain-balanced with strain in the opposite direction in adjacentsemiconductor material.

The present disclosure also describes a semiconductor structure that, insome implementations, achieves greater semiconductor device currentdensity, voltage, fill factor, and/or energy conversion efficiency dueto one or more improved transport (IT) layer(s) incorporated into thesemiconductor device such as a solar cell or photodetector, where theimproved charge-carrier transport in the layer is achieved by theelimination, reduction, addition, or increase of one or more elements inthe semiconductor chemical composition of the IT layer, alteration ofthe sublattice ordering in the IT layer, alteration of the compressiveor tensile strain in the IT layer, reduction of the density of crystaldefects in the IT layer, reduction of impurities in the IT layer, and/oralteration of the type of crystal defects in the IT layer, where theabove changes in the IT layer are made relative to the primaryphotogeneration layers, adjacent photogeneration layers, othersemiconductor layers, or the bulk of other semiconductor layers in thesemiconductor device. Improved transport (IT) layer(s) are defined aslayer(s) with higher minority-carrier lifetime, minority-carriermobility, minority-carrier diffusion length, majority-carrier mobility,majority-carrier conductance, charge-carrier saturation velocity in anelectric field, and/or collected photogenerated current density than inthe primary photogeneration layers, adjacent photogeneration layers,other semiconductor layers, or the bulk or other semiconductor layers inthe semiconductor device.

The low-bandgap absorber region (LBAR) layer or layers described hereinmay or may not also function as improved transport (IT) layer(s), andthe IT layer or layers described herein may or may not also function asLBAR layer(s).

LBARs and/or IT layers may be positioned in different regions of a solarcell, such as in the quasi-neutral region of the emitter of a solarcell, in the space-charge region of the emitter, in the space-chargeregion of the base of a solar cell, in the quasi-neutral region of thebase, in the space-charge region of an isotype junction formed bysemiconductors of the same doping type but with different carrierconcentrations and/or different semiconductor compositions(heterojunction), in the quasi-neutral region of a window layer, in thequasi-neutral region of a back-surface field (BSF) layer, and elsewherein a solar cell, as well as in combinations of these solar cell regions.Further, LBARs and/or IT layers may be positioned in a lattice-matcheddevice, (e.g., one in which the majority of the semiconductor materialin the device has the same material lattice constant as a growthsubstrate, other layers, and/or other devices grown in the samesemiconductor stack), or may be positioned in a lattice-mismatcheddevice, (e.g., one in which the majority of the semiconductor materialin the device has a different material lattice constant than that of agrowth substrate, other layers, and/or other devices grown in the samesemiconductor stack). LBARs and/or IT layers may be positioned invarious functional elements of a semiconductor device. For example,LBARs and/or IT layers may be positioned in the emitter, base, window,back surface field (BSF) layer, or other layers of a solar cell,photodetector, or other type or optoelectronic device. LBARs and/or ITlayers may be positioned in a single junction solar cell, in a subcellof a multijunction solar cell, in a photodetector, or in other types ofoptoelectronic devices. LBARs and/or IT layers may be positioned in alattice-matched subcell of a multijunction cell in which all othersubcells are also lattice-matched, or in a lattice-matched subcell of amultijunction cell in which one or more of the other subcells islattice-mismatched, or metamorphic, or in a lattice-mismatched ormetamorphic subcell of a multijunction solar cell. LBARs and/or ITlayers may have a 0-dimensional (0D), 1-dimensional (1D), 2-dimensional(2-D), or 3-dimensional (3D) structure.

LBARs introduced into the solar cell structure may, and often do haveimproved minority-carrier transport properties, such as improvedminority-carrier lifetime, mobility, and diffusion length, which in turnimprove the probability of current collection in the solar cell beforethe minority carriers can recombine. Thus in these cases, the LBARs alsofunction as improved transport (IT) layers.

The improved minority-carrier transport properties in IT layers, whichmay also function as LBARs, may result from: different compositionleading to lower impurity or defect density, such as the elimination,reduction, addition, or increase of one or more chemical elements in acompound semiconductor; different composition leading to lower carriereffective mass and higher carrier mobility; different strain states;different sublattice ordering states; reduced incorporation of chemicalimpurities, reduced density of crystal lattice defects, different typesof crystal lattice defects, and other phenomena in the improvedtransport (IT) region compared to other parts of the solar cell.Moreover, these improved transport regions may have improvedmajority-carrier transport properties, such as majority-carrier mobilityand conductance, which may result from the differences listed above.Furthermore, these improved transport regions, defined as regions withbetter minority-carrier and/or better majority carrier propertiescompared to other parts of the solar cell, may be low bandgap absorberregions (LBARs), but in other cases may also be regions that actuallyhave higher bandgap, or the same bandgap, as adjacent portions of thesolar cell, and still be within the definition of the presentdisclosure.

The LBARs or improved transport (IT) regions may be used in one or moresubcells within a multijunction solar cell, where a multijunction cellrefers to a solar cell having 2 or more junctions. Because of thelimited availability of semiconductors with the optimum bandgaps to formsubcells that will result in the highest multijunction solar cellefficiency; because of the practical limitations on minority- andmajority-carrier transport parameters in semiconductors that areotherwise suitable in terms of their bandgap and lattice constant; andbecause of the limited availability of semiconductors with the desiredlattice constants to allow semiconductor crystal growth with a minimumof defects that increase minority-carrier recombination, it is oftenhighly desirable to increase the current collection probability in oneor more subcells of a multijunction solar cell, through the use of LBARsor improved transport (IT) regions. For example, photogenerated currentdensity may be increased by the use of an LBAR in the top subcell, alsoreferred to as cell 1 (C1), subcell 1, top cell, upper cell, or uppersubcell, of a multijunction cell. Further, the collected current densityin response to light may be increased by the use of an IT layer in thetop subcell, for example, in the emitter of the top subcell, and an LBARin the top cell may also be an IT layer.

Similarly, because of these constraints of limited availability ofsemiconductors with the optimum bandgaps; practical limitations onminority- and majority-carrier transport parameters; and limitedavailability of semiconductors with the desired lattice constants with aminimum of defects, it is often highly desirable to extend thecurrent-producing capability of one or more subcells of a multijunctionsolar cell to longer wavelengths through the use of LBARs or improvedtransport (IT) regions.

In multijunction solar cells it is often desirable to increase thebandgap of the top subcell, also referred to as cell 1 (C1), also calledsubcell 1, top cell, upper cell, or upper subcell, of the multijunctionstack, so that photogenerated current in the top cell can be collectedat a higher voltage for greater power output. The higher top cellbandgap means that it will not be able to use the longer wavelengths inits former response range, but those wavelengths will be transmitted tocell 2 (C2), also called subcell 2, beneath the top cell, therebyincreasing the current density of cell 2.

In practical designs the top cell may be optically thin in order to leaklight through to cell 2 in order to match the currents of cell 1 andcell 2. Increasing the cell 1 bandgap allows the multijunction celldesigner to increase the cell 1 thickness and still achieve the samecell 1 current for current matching, while actually reducing the amountshort wavelength light that is transmitted through cell 1 to cell 2.This not only allows that shorter wavelength light to be used at thehigher voltage of cell 1, but also reduces unwanted absorption that doesnot result in current that can be collected usefully in layers betweensubcells, such as tunnel junction layers that interconnect the subcells,as well as in subcell layers such as back surface field (BSF) or windowlayers where minority-carrier properties may be less favorable than inother parts of the subcell. Since the cell 1 thickness is greater, andthe use of an extremely thin cell 1 for very high optical transparencyis avoided, it can also make it much easier to control the thickness andcurrent generation in cell 1, and thereby to control the current balancebetween cell 1 and cell 2 for optimum current output and efficiency.

In order to increase the bandgap of the top cell, aluminum (Al) is oftenadded to the top cell semiconductor composition, or the Al compositionis increased, e.g., to form an AlGaInP, AlGaAs, AlGaInAs, AlInAs, orAlGaInPAs top cell. Since Al reacts strongly with oxygen (O), morestrongly than other group-III elements such as Ga and In react withoxygen, Al atoms tend to react with even small amounts of H₂O and O₂present in the epitaxial growth chamber or in sources used for epitaxialgrowth, e.g., metal-organic sources such as trimethylaluminum (TMAl) andtrimethylindium (TMIn), so that oxygen atoms, and potentially otherimpurities, are unintentionally incorporated into the semiconductormaterial in higher concentrations when the Al composition is higher.These Al-induced impurities, and perhaps other Al-related defects aswell, tend to increase minority-carrier recombination in Al-containingsemiconductors, e.g., AlGaInP, AlGaInAs, or AlGaInSb, and decreaseminority-carrier lifetime, mobility, and diffusion length, and candecrease majority-carrier mobility and conductivity, relative to similarsemiconductors with no Al content, i.e., Al-free semiconductors, e.g.,GaInP, GaInAs, or GaInSb, or to similar semiconductors with reduced Alcontent.

Other methods to increase the bandgap of the top cell include usingcompositions with a lower In/Ga ratio. Lowering the In/Ga ratio in thetop cell semiconductor composition, e.g., in a top cell with a GaInPbase, also changes the semiconductor lattice constant, which may beacceptable, but the tendency for dislocations to form in thicklattice-mismatched layers without a suitable metamorphic buffer totransition from one lattice constant to another must be accounted for inthe device design.

The present disclosure describes, in some implementations, asemiconductor device structure having higher minority-carrier lifetime,mobility, and diffusion length, and/or higher majority-carrier mobilityand conductance. As described in greater detail herein, this structurewith enhanced charge-carrier transport properties, or improved transport(IT) region, may be a low-bandgap absorber region (LBAR), or maycomprise one or more LBARs. This improved transport (IT) region, inother cases, may instead have a bandgap greater than or equal to that ofother nearby regions in the solar cell, and therefore may be an improvedtransport (IT) region without being a low-bandgap absorber region(LBAR), and still be included in the present disclosure. The improvedtransport (IT) region device structure with enhanced charge-carriertransport properties may improve the voltage, current, fill factor, orcombinations of these parameters in a solar cell or other optoelectronicdevice.

In one implementation, it is desirable to achieve higherminority-carrier lifetime, mobility, and diffusion length, and highermajority-carrier mobility and conductance, by incorporating an Al-freeemitter, e.g., a GaInP emitter, or a reduced-Al-content emitter, in asolar cell with a base that contains Al, e.g., a solar cell with a basecomposed of AlGaInP. This forms an LBAR in the quasi-neutral region ofthe solar cell emitter.

In an implementation, it can be advantageous to achieve greater currentdensity from photogeneration in an LBAR in the quasi-neutral region ofthe emitter of a subcell, formed by an aluminum-free emitter such asGaInP, or an emitter with reduced aluminum content relative to the base,in a solar cell with an aluminum-containing base such as AlGaInP. Thephrase “AlGaInP subcell” is taken to mean a subcell which has a base—orother region or the solar cell which produces the largest amount ofphotogenerated current density in the device, termed the primaryphotogeneration region of the solar cell, such as an emitter that isthicker than the base layer—which is composed of AlGaInP. Similarly thephrase “(composition X) subcell” is taken to mean a subcell with a baseor other primary photogeneration region that has semiconductorcomposition X. The greater current density from the LBAR may beadvantageous if the effect of any voltage decrease, which maypotentially result from the lower bandgap of the LBAR, on the poweroutput of the solar cell is outweighed by the increased current densityresulting from the lower bandgap and greater photogeneration in theLBAR. The greater current density from the LBAR may also be advantageousfor balancing the photogenerated current densities of subcells in aseries-interconnected multijunction stack, where other factors mayrestrict current balancing by more conventional means, such as changingthe thickness of the solar cell, or by changing the bandgap of both theentire base and emitter of the solar cell.

Subcells may have AlGaInP forming part or all of the subcell base, withother parts of the solar cell such as the emitter and other parts of thebase composed of GaInP or lower-Al-content AlGaInP, in order to retainthe better minority-carrier recombination properties, and betterminority-carrier and majority-carrier mobility properties, that GaInP orlower-Al-content AlGaInP often have in practice compared to AlGaInP. TheGaInP or lower-Al-content AlGaInP layer with improved minority-carrierand/or majority-carrier properties may make take up the entire width ofthe solar cell emitter, so that it constitutes the solar cell emitter,or may make up only part of the emitter region. The GaInP orlower-Al-content AlGaInP layer with improved minority-carrier and/ormajority-carrier properties may also make up part or all of the solarcell base or of other layers in the solar cell.

LBARs of general composition and construction may be placed in theemitter region of a solar cell. The LBAR or LBARs may be placed in thespace-charge region associated with the solar cell emitter, in thequasi-neutral region of the solar cell emitter, partly in thespace-charge region and partly in the quasi-neutral region of the solarcell emitter. The one or more LBARs may take up the entire width of thesolar cell emitter, so that the one or more LBARs constitute the solarcell emitter. In general the LBAR or LBARs may have portions in thequasi-neutral region of the emitter, space-charge region of the emitter,space-charge region of the base, and/or in the quasi-neutral region ofthe base. The collection of carriers will be aided by the electric fieldin the space-charge region, but current collection from carrierdiffusion can still be quite appreciable and beneficial for LBARspositioned in the quasi-neutral regions, particularly those portions ofthe quasi-neutral regions adjacent to the space-charge region.

LBARs have higher photogeneration by virtue of their lower bandgap thansurrounding semiconductor layers, and charge carriers may leave the LBARby thermal escape, tunneling through barriers, and/or field-assistedescape. When the charge carriers escape the LBARs, and are collected,e.g., at a p-n junction, the LBARs increase the current density of thesolar cell. The lower bandgap of the LBARs compared to surroundingsemiconductor material allows them to make use of lower energy photonsin the incident light spectrum to produce useful current, than the solarcell or other optoelectronic device would otherwise be able to use, forexample: because it would require a metamorphic or lattice-mismatchedsolar cell composition with a tendency to have a greater number ofdislocations and undesirable carrier recombination centers which lowerminority-carrier lifetime and solar cell voltage; because of theunavailability of semiconductor materials with high-quality bulkproperties for the needed composition with the desired bandgap; orbecause of the expense of the materials. LBARs in one or more of thesubcells of a multijunction cell may be used to increase the current ofthat subcell beyond that which would be practical, convenient, orcost-effective in a solar cell without LBARs, allowing the wavelengthresponses of the multijunction cell to more closely approximate theideal division of the solar spectrum for high-efficiency energyconversion, and to be current matched in a series-interconnectedmultijunction cell.

LBARs may be incorporated into arrays with layers having alternatinglarger and smaller material lattice constant to form a strain-balancedstructure, e.g. of multiple pseudomorphic tensile energy barriers andcompressive energy wells, or LBARs, to increase the optical thicknessand current generating ability of the LBARs without allowing the lowbandgap, large lattice constant semiconductor layers to relax theircrystalline structure, forming dislocations that are highly activerecombination sites, as they may in very thick layers withoutintervening tensile strain-balancing layers. LBARs may also be formedwith no strain with respect to the surrounding material, or in tensilestrain with respect to the surrounding material. When unstrained, LBARsmay be formed in thick layers, or regions large enough to havenegligible quantum effects on the energy levels of charge carriers inthe LBARs or energy wells, and may be formed without accompanyingstrain-balance layers or barrier layers. When the strain is relativelysmall, LBARs may also be formed in thick layers, or regions large enoughto have negligible quantum effects on the energy levels of chargecarriers in the LBARs or energy wells, and may be formed with or withoutaccompanying strain-balance layers or barrier layers, and remainpseudomorphic.

LBARs may have a bandgap that is lower than: the bandgap of thesemiconductor material elsewhere in the device; the bandgap of the bulkof photogeneration or light-absorbing material in a solar cell or otheroptoelectronic device such as a multijunction solar cell or othermultijunction optoelectronic device; the bandgap of the bulk ofphotogeneration or light-absorbing material in the base and/or emitterof a solar cell or other optoelectronic device; and/or the bandgap oflayers disposed between the LBARs. The layers disposed between the LBARsmay be strain-balance layers, i.e., layers that balance the strainintroduced by the LBARs themselves with strain in the oppositedirection. The LBARs may be strained or unstrained with respect to theother photogeneration or light-absorbing materials in a solar cell orother optoelectronic device, and may be strained or unstrained withrespect to any layers that may be disposed between the LBAR layers.Layers between the LBARs may have a smaller or larger material latticeconstant than the material lattice constant of the LBARs, where thematerial lattice constant of a semiconductor is the unstrained latticeconstant for that semiconductor composition. Layers between the LBARsmay have a bandgap higher than, lower than, or equal to that of the bulkof photogeneration or light-absorbing material in a solar cell or otheroptoelectronic device.

The LBARs may have a variety of geometrical configurations, influencedby ease of manufacture and/or ability to produce the desired effect ofincreasing current density or other effect in the solar cell or otheroptoelectronic device. Multiple LBARs may be used in the same solar cellor other device, including to increase light absorption by the LBARs.

In addition to other structures described herein, LBARs describedherein, in some implementations, can comprise a quantum confinedstructure. For example, in some cases, one or more LBARs may be2-dimensional (2-D) sheets or layers, 1-dimensional (1-D) linearfeatures, or 0-dimensional (0-D) point-like (dot) features. The LBARs,whether 2-D, 1-D, or 0-D features, may have size scales small enoughthat the confined carriers show a change in energy level due to quantummechanical effects (quantum confinement), or may have size scales largeenough that the confined carriers have a small or negligible change inenergy level, where a small or negligible change may be considered to beless than or equal to ½ kT (0.5 kT), where k is the Boltzmann constantand T is the absolute temperature in kelvins. For example, quantum wells(2-D), quantum wires (1-D), or quantum dots (0-D) may be used, or in theother extreme, relatively large regions in comparison to the quantumconfinement distance scale, or the entirety of the space charge region,base quasi-neutral region, and/or emitter quasi-neutral region, andcombinations thereof, may be of a lower bandgap than the rest of thecell, in a sheet-like (2-D), rod-like (1-D), or granule-like (0-D)configuration in which energy levels are not shifted or are shifted onlyto a small degree due to quantum mechanical effects.

In one implementation, a single junction solar cell with unstrained lowbandgap absorber regions (LBARs), or multijunction solar cell in whichat least one of the subcells has unstrained low bandgap absorber regions(LBARs), is disclosed, where the well material and the barrier materialor other parts of the solar cell with larger bandgap than the LBARs havethe same lattice constant but different bandgaps. For example, someexemplary structures include the following:

-   1. ordered GaInP LBAR(s) or energy well(s) with adjacent disordered    GaInP barrier(s), or other parts of the solar cell with higher    bandgap than the LBAR(s), e.g., the base or emitter of a solar cell,    at a lattice constant between that of GaP and InP, including at the    GaAs or Ge lattice constant;-   2. zero-Al-% or low-Al-% (Al)GaInP LBAR(s) or energy well(s) with    adjacent high-Al-% AlGaInP barrier(s), or other parts of the solar    cell with higher bandgap than the LBAR(s), e.g., the base or emitter    of a solar cell, at a lattice constant between that of GaP and InP,    including at the GaAs or Ge lattice constant;-   3. zero-Al-% or low-Al-% (Al)GaInAs LBAR(s) or energy well(s) with    adjacent high-Al-% AlGaInAs barrier(s), or other parts of the solar    cell with higher bandgap than the LBAR(s), e.g., the base or emitter    of a solar cell, at a lattice constant between that of GaAs and    InAs, including at the GaAs, Ge, or InP lattice constant;-   4. zero-P-% or low-P-% GaIn(P)As LBAR(s) or energy well(s) with    adjacent high-P-% GaInPAs barrier(s), or other parts of the solar    cell with higher bandgap than the LBAR(s), e.g., the base or emitter    of a solar cell, at a lattice constant between that of GaAs and    InAs, including at the GaAs, Ge, or InP lattice constant;-   5. zero-In-% or low-In-% Ga(In)PAs LBAR(s) or energy well(s) with    adjacent high-P-% GaInPAs barrier(s), or other parts of the solar    cell with higher bandgap than the LBAR(s), e.g., the base or emitter    of a solar cell, at a lattice constant between that of Si and GaAs,    including at the Si, GaP, or GaAs lattice constant; and-   6. Ga(In)NAs(Sb) LBAR(s) or energy well(s) with adjacent    Ga(In)(N)(P)As(Sb) barrier(s), or other parts of the solar cell with    higher bandgap than the LBAR(s), e.g., the base or emitter of a    solar cell, at a lattice constant between that of GaAs and GaSb,    including at the GaAs, Ge, InP, InAs, or GaSb lattice constant.

In one implementation, a multijunction solar cell in which at least oneof the subcells has strain-balanced low band-gap absorber regions(LBARs) is disclosed. The barrier material and the well material arestrain-balanced such that the average lattice constant of the well andbarrier material, weighted by their thicknesses, is the same or nearlythe same as the lattice constant of other parts of the cell. Forexample, some exemplary structures include the following:

-   1. high-In-% GaInAs LBAR(s) or energy well(s) with adjacent zero-In    % or low-In-% Ga(In)As barrier(s), or other parts of the solar cell    with higher bandgap than the LBAR(s), e.g., the base or emitter of a    solar cell, and at a lattice constant between that of GaAs and InAs,    including at the GaAs, Ge, InP, or InAs lattice constant;-   2. GaInAs LBAR(s) or energy well(s) with adjacent GaPAs barrier(s),    or other parts of the solar cell with higher bandgap than the    LBAR(s), e.g., the base or emitter of a solar cell, and at a lattice    constant between that of GaAs and InAs, including at the GaAs, Ge,    InP, or InAs lattice constant;-   3. low-P-% GaPAs LBAR(s) or energy well(s) with adjacent high-P-%    GaPAs barriers, or other parts of the solar cell with higher bandgap    than the LBAR(s), e.g., the base or emitter of a solar cell, at a    lattice constant between that of Si and GaAs, including at the Si,    GaP, or GaAs lattice constant;-   4. GaIn(P)As LBAR(s) or energy well(s) with adjacent Ga(In)PAs    barrier(s), or other parts of the solar cell with higher bandgap    than the LBAR(s), e.g., the base or emitter of a solar cell, and at    a lattice constant between that of Si and InAs, including at the Si,    GaP, GaAs, Ge, InP, or InAs lattice constant;-   5. high-In-% GaInP LBAR(s) or energy well(s) with adjacent low-In-%    GaInP barriers, or other parts of the solar cell with higher bandgap    than the LBAR(s), e.g., the base or emitter of a solar cell, at a    lattice constant between that of Si and GaAs, including at the Si,    GaP, or GaAs lattice constant;-   6. high-In-% GaInP LBAR(s) or energy well(s) with adjacent low-In-%    GaInP barriers, or other parts of the solar cell with higher bandgap    than the LBAR(s), e.g., the base or emitter of a solar cell, at a    lattice constant between that of GaAs and InP, including at the    GaAs, Ge, or InP lattice constant;-   7. high-Sb-% GaAsSb LBAR(s) or energy well(s) with adjacent low-Sb-%    GaAsSb barriers, or other parts of the solar cell with higher    bandgap than the LBAR(s), e.g., the base or emitter of a solar cell,    at a lattice constant between that of GaAs and GaSb, including at    the GaAs, Ge, InP, InAs, or GaSb lattice constant; and-   8. Ga(In)NAs(Sb) LBAR(s) or energy well(s) with adjacent    Ga(In)(N)(P)As(Sb) strain balance layer(s), barrier(s), or other    parts of the solar cell with higher bandgap than the LBAR(s), e.g.,    the base or emitter of a solar cell, at a lattice constant between    that of GaAs and GaSb, including at the GaAs, Ge, InP, InAs, or GaSb    lattice constant.

If desired, in some implementations comprising a plurality of LBARlayers, the LBAR layers may have differing thicknesses from one another.Furthermore, one or more strained or barrier layers may have differingthickness form one another. For example, an array of (n−1) LBARs ofthickness x may be interleaved with n strained or barrier layers forstrain balance of thickness y, where n is an integer, and additionalLBARs of thickness x/2 may be placed at each end of the array of LBARsand strained or barrier layers, to complete the strain balance of theoverall array. In another example, an array of n LBARs of thickness xmay be interleaved with (n−1) strained or barrier layers for strainbalance of thickness y, where n is an integer, and additional strainedor barrier layers of thickness y/2 may be placed at each end of thearray of LBARs and strained or barrier layers, to complete the strainbalance of the overall array.

Moreover, a semiconductor structure described herein may includediffering numbers of LBAR layers and strained or barrier layers. Forexample, an array of n LBARs of thickness x may be interleaved with(n−1) strained or barrier layers for strain balance of thickness y,where n is an integer, where the LBAR and strained or barrier layerthicknesses and compositions are tuned in order to strain balance theoverall array, such that the LBARs are on the outside of the array,which may help with carrier escape in some instances, and/or may help toimprove semiconductor interface quality for some layer compositions. Inanother example, an array of n LBARs of thickness x may be interleavedwith (n+1) strained or barrier layers for strain balance of thickness y,where n is an integer, where the LBAR and strained or barrier layerthicknesses and compositions are tuned in order to strain balance theoverall array, such that the strained or barrier layers are on theoutside of the array, which may help to suppress minority-carrierconcentration and recombination at interfaces in some instances, mayhelp to confine charge carriers, may help to confine, increase, decreasedopant species concentration, may help to suppress diffusion of dopantatoms, and/or may help to improve semiconductor interface quality forsome layer compositions.

In some instances, LBARs with lower bandgap than the solar cell base oremitter have a composition which also causes them to have a largermaterial lattice constant than, and to be in compressive strain withrespect to the solar cell base or emitter. Similarly,strain-compensation regions (SCRs)—with a material lattice constant lessthan, and tensile strain with respect to the solar cell base or emitterand to the LBARs, designed to strain-balance the compressive LBARs—havea composition which also causes them to have higher bandgap than thesolar cell base and than the LBARs.

Thus, in some implementations, a single LBAR in compressive strain mayhave its strain compensated, or be strain balanced, by a single straincompensation region (SCR) in tensile strain, with a larger latticeconstant than the LBAR. In one implementation, the emitter of a solarcell may be an LBAR in compressive strain, for which the strain iscompensated by a window layer in tensile strain with a higher bandgapthan the emitter LBAR. In an implementation, the emitter of a solar cellmay have a composition such that it is in tensile strain with a higherbandgap than for an unstrained composition of the same semiconductorfamily, and the window of a solar cell may have a composition such thatit is in compressive strain, strain-balancing the tensile strain of theemitter. In an implementation, the window layer which strain-compensatesthe strain in the emitter may have a bandgap that is higher than, lowerthan, the same as, or nearly the same as the bandgap of the emitterlayer. When the emitter and/or window have higher bandgap than theywould for their lattice-matched, unstrained compositions, an advantageis that the light transmission of those layers is increased to layersbeneath that are better able to use light at the transmittedwavelengths. In an implementation, multiple LBARs in compressive strainwith bandgap lower than the solar cell layer into which they areinserted may alternate in position with SCRs in tensile strain withbandgap higher than the LBARs and higher than the solar cell layer intowhich they are inserted.

It need not always be the case that LBARs, with low bandgap compared tothe solar cell layer in which they are imbedded, are in compressivestrain, and it need not always be the case that SCRs in tensile strainhave higher bandgap than the solar cell layer in which they areimbedded, and the LBARs which they strain compensate. In animplementation, LBARs with a low bandgap for enhanced photoabsorptionand photocurrent generation and relatively large material latticeconstant may be strain-compensated with SCRs with relatively smallmaterial lattice constant which also have low bandgap for enhancedphotoabsorption and photocurrent generation. In an implementation, bothLBARs and SCRs are incorporated into a solar cell base, and both LBARsand SCRs have bandgaps that are lower than that of the solar cell base.In an implementation, the LBARs have larger material lattice constantthan the SCRs, and are in compressive strain with respect to the SCRswithin the array of LBARs and SCRs, while the SCRs have smaller materiallattice constant than the LBARs, and are in tensile strain with respectto the LBARs within the array of LBARs and SCRs. In an implementation,an extended low bandgap absorber region may be formed from alternatingcompressive and tensile LBAR and SCR layers, all of which have lowbandgap for enhanced photoabsorption and photocurrent generation, wherethe extended low bandgap absorber region has a thickness larger, andpotentially much larger than the thickness at which either the strainedLBAR material or the strained SCR material could remain pseudomorphic,with a coherent, high-quality, low-dislocation, single crystallinestructure. In an implementation, this extended low bandgap absorberregion comprising alternating compressive and tensile low bandgap layersforms part of the solar cell base or emitter, for instance, a part ofthe base or emitter within the space-charge region near the p-njunction, a part of the emitter quasi-neutral region, a part of the basequasi-neutral region, or combinations thereof. In an implementation,this extended low bandgap absorber region comprising alternatingcompressive and tensile low bandgap layers forms all of the solar cellbase or emitter. In other implementations, the LBAR and/or SCR layersdescribed above as being incorporated into a solar cell base and/oremitter may also be incorporated into the back-surface field (BSF)layer, window, and/or any other layer in the structure of a solar cell,or any combination of the above layers of a solar cell structure. Inother implementations, the LBAR and/or SCR layers with bandgaps higher,lower, or the same as a solar cell base and/or emitter, may also bedesigned with bandgaps higher, lower, or the same as the BSF layer,window, or any other layer in the structure of a solar cell, or anycombination of the above layers of a solar cell structure.

When layers with smaller lattice constant than that of the solar cellbase are used to counteract the compressive strain of 2D (sheets), 1D(wires), or 0D (dots) LBARs with larger lattice constant than that ofthe solar cell base, those small-lattice-constant strain-compensationlayers tend to have higher bandgap than that of the LBARs or the solarcell base, since within the same or similar semiconductor materialsystem there is a tendency for bandgap to rise as one alters thecomposition to lower the lattice constant. This higher bandgap of thestrain-compensation layers between the LBARs may be detrimental to solarcell performance, since 1) these higher bandgap regions limit the numberof LBARs and their cumulative thickness available for light absorptionthat can be placed in a given thickness of the solar cell, e.g., inspace-charge regions or quasi-neutral regions of a solar cell; and 2)higher bandgap layers are barriers for carrier transport that can blockthe flow of useful current photogenerated in the LBAR to the collectingp-n junction and to the solar cell terminals.

However, some semiconductors exhibit a decrease in bandgap as thelattice constant is decreased, e.g., dilute nitride GaNAs, GaInNAs, andGaInNAsSb semiconductor compositions with 0 to 5 atomic percentnitrogen, and more preferably 0.5-3.0% nitrogen. This allows theLBAR/strain-compensation layer structure to be engineered such that boththe LBARs and the SCRs have a lower bandgap than the solar cell base,thus maximizing the thickness available for light absorption andincreased current photogeneration, and removing high bandgap barriers tocurrent flow. The bandgap in the LBARs and SCRs may be madeapproximately equal to each other if desired.

In an implementation, a solar cell may incorporate LBARs composed ofGaInAs with low bandgap for enhanced photocurrent generation andcompressive strain, and may incorporate SCRs composed of GaNAs whichalso have low bandgap for enhanced photocurrent generation, but whichhave smaller material lattice constant than that of the GaInAs LBARs,and which have tensile strain in order to compensate the strain of theLBARs, thus forming a pseudomorphic, extended low bandgap absorberregion which is much thicker than the maximum thickness at which eitherthe strained LBAR material or the strained SCR material can remainpseudomorphic.

A thick low bandgap layer may also be produced by allowing the elementsof the LBAR and the SCRs to form a homogeneous LBAR with the samelattice constant as the solar cell base, for instance, an LBAR formedfrom GaInNAs with a lower bandgap, but the same lattice constant as aGaInAs solar cell base. It may be desirable in some cases to separatecertain elements from others, to avoid undesirable interactions in thecrystal lattice that cause increased recombination, e.g. the formationof In—N pairs in GaInNAs, or to avoid gas phase reactions during growth,by introducing reactive precursor gases into the reaction chamber atdifferent times. This can be accomplished by using the LBAR/SCRstructure described above, e.g., with high-In GaInAs LBARs combined withGaNAs SCRs, achieving a low bandgap region, but with lower recombinationrate due to the separation of In and N in their respective layers.

LBARs may have a wide range of spatial extents: they may be quite widewith widths from 0.1 micron to about 1 micron or more; they may be in amiddle range with widths from approximately 100 Angstroms (Å) to about1000 A, or they may be quite narrow in a range from greater than 0 toabout 100 A in which there is a strong effect of quantum confinement onthe energy levels of carriers within the LBARs. The width of the LBARsand the SCRs is primarily determined by the constraints of howlattice-mismatched the layers are from each other and the solar cellbase, and how thick each layer can become while remaining pseudomorphic,i.e., before the crystal lattice relaxes and develops dislocations.

In the case of GaInAs LBARs and GaNAs SCRs, the LBARs may be relativelythick, or they may be extremely thin with thickness on the order of oneto several lattice constants, to form a metamaterial of GaInAs and GaNAslayers with properties different from a homogeneous alloy of GaInNAs.These same concepts can be extended to other semiconductor materialsystems as well.

In another implementation the tensile-strained layer or layers and thecompressive strained layer or layers incorporated into the solar cellstructure, may both have bandgaps higher than that of the solar cellbase, emitter, BSF layer, window, or other solar cell structure orcombination of structures. In some instances and in some solar cellstructures, the reduced minority-carrier recombination that can resultfrom the higher bandgap of these layers or layers may be more desirablethan the increased photogenerated current that can result from a lowerbandgap layer, and may thus result in an improved transport (IT) layer.In another implementation the tensile-strained layer or layers and thecompressive strained layer or layers incorporated into the solar cellstructure, may both have bandgaps the same as that of the solar cellbase, emitter, BSF layer, window, or other solar cell structure orcombination of structures.

In another implementation the tensile-strained layer or layers may havea bandgap or bandgaps that is/are higher than that of the solar cellbase, emitter, BSF layer, window, or other solar cell structure orcombination of structures, while the compressive strained layer orlayers incorporated into the solar cell structure may have a bandgap orbandgaps that is/are lower than that of the solar cell base, emitter,BSF layer, window, or other solar cell structure or combination ofstructures. In another implementation the tensile-strained layer orlayers may have a bandgap or bandgaps that is/are lower than that of thesolar cell base, emitter, BSF layer, window, or other solar cellstructure or combination of structures, while the compressive strainedlayer or layers incorporated into the solar cell structure may have abandgap or bandgaps that is/are higher than that of the solar cell base,emitter, BSF layer, window, or other solar cell structure or combinationof structures.

In another implementation the tensile-strained layer or layers and/orthe compressive strained layer or layers may fully replace the solarcell base, emitter, BSF layer, window, or other solar cell structure orcombination of structures. In another implementation thetensile-strained layer or layers and/or the compressive strained layeror layers may replace part of, or be incorporated into, the solar cellbase, emitter, BSF layer, window, or other solar cell structure orcombination of structures.

Low bandgap absorber regions, or LBARs, can also be formed frommaterials that are unstrained or have little strain with respect to thesolar cell semiconductor structure in which they are incorporated, e.g.,the solar cell base. In this case, SCRs are not needed to balance thestrain of the LBARs, since they have no strain or little strain. Forthis situation, very wide LBARs can be formed, since there is no longerany maximum thickness constraint for the material to remainpseudomorphic in the unstrained case, or this thickness is very large inthe case with very small strain. This enhances the ability of the LBARto absorb light and generate current since the thickness is not limitedby strain concerns. Even within finite thicknesses imposed by otherfeatures in the cell, such as the width of the space charge region widthin the case for which it is desired to have the LBAR within the spacecharge region, this allows greater photoabsorption, photogeneration ofelectron-hole pairs, and photocurrent collection, since all of thisthickness can be taken up with the LBAR, with none of it consumed withhigh bandgap SCRs.

In an implementation, a solar cell may incorporate an LBAR or LBARswhich have the same material lattice constant as the solar cellstructures in which they are incorporated, and thus have no strain orlittle strain, and thus require no SCRs. In an implementation, the LBARor LBARs have the same composition and same material lattice constant asthe solar cell base, but are lower in bandgap due to a greater amount ofatomic ordering of two or more elements in the semiconductor crystalstructure. In an implementation, the LBAR or LBARs have the samecomposition and same material lattice constant as the solar cell base,but are lower in bandgap due to a greater amount of atomic ordering onthe group-III sublattice of the semiconductor crystalline structure inthe LBAR or LBARs. In an implementation, the LBAR or LBARs have the samecomposition and same material lattice constant as the solar cell base,but are lower in bandgap due to a greater amount of atomic ordering onthe group-V sublattice of the semiconductor crystalline structure in theLBAR or LBARs.

In an implementation, the LBAR or LBARs are composed of GaInP with thesame composition and same material lattice constant as a GaInP solarcell base, but with lower bandgap in the LBAR or LBARs due to an orderedor partially ordered arrangement of Ga and In atoms on the group-IIIsublattice, thus lowering their bandgap with respect to a disordered orpartially disordered GaInP base. Since the GaInP LBARs are of the sameor similar composition as the GaInP base, SCRs are not needed for strainbalancing.

In an implementation, a solar cell may incorporate an LBAR or LBARswhich have the same or a similar lattice constant to the solar cellstructure into which they are incorporated, such as a solar cell base,but have lower bandgap due to a different semiconductor composition inthe LBAR (or LBARs) and the solar cell base. Again, since the latticeconstant is the same or similar in the LBAR (or LBARs) and in the solarcell base, no SCRs are needed for strain compensation.

In an implementation, LBARs with GaInP or lower-Al AlGaInP compositionmay be incorporated into a solar cell base with higher-Al AlGaInPcomposition, with the same or similar material lattice constant in theLBAR or LBARs as in the solar cell base. In an implementation, LBARswith GaAs or lower-Al AlGaAs composition may be incorporated into asolar cell base with higher-Al AlGaAs composition, with the same orsimilar material lattice constant in the LBAR or LBARs as in the solarcell base. In an implementation, LBARs with GaAs, GaInAs, or lower-AlAlGaInAs composition may be incorporated into a solar cell base withhigher-Al AlGaInAs composition, with the same or similar materiallattice constant in the LBAR or LBARs as in the solar cell base. In animplementation, LBARs with GaInAs or GaInPAs composition may beincorporated into a solar cell base with GaInPAs or GaInP composition,with the same or similar material lattice constant in the LBAR or LBARsas in the solar cell base. In an implementation, LBARs with GaN orlower-Al AlGaN composition may be incorporated into a photovoltaic cellbase with higher-Al AlGaN composition, with the same or similar materiallattice constant in the LBAR or LBARs as in the photovoltaic cell base.In an implementation, LBARs with GaSb or lower-Al AlGaSb composition maybe incorporated into a solar cell base with higher-Al AlGaSbcomposition, with the same or similar material lattice constant in theLBAR or LBARs as in the solar cell base. In an implementation, LBARswith GaInAs or GaInAsSb composition may be incorporated into a solarcell base with GaInAsSb or GaAsSb composition, with the same or similarmaterial lattice constant in the LBAR or LBARs as in the solar cellbase. In an implementation, LBARs with InAsSb or GaInAsSb compositionmay be incorporated into a solar cell base with GaInAsSb or GaInSbcomposition, with the same or similar material lattice constant in theLBAR or LBARs as in the solar cell base.

In an implementation, a solar cell or photovoltaic cell may incorporatesmall-lattice-constant strain-compensation regions (SCRs) such that thematerial lattice constant and thickness of the SCRs strain balances thestrain in the LBAR layers, so that all layers may remain pseudomorphicwith high crystal quality at the same in-plane lattice constant. In animplementation, the solar cell incorporates only LBARs, without SCRs. Inan implementation, the solar cell incorporates only SCRs, without LBARs.In an implementation, the solar cell incorporates both LBARs and SCRs.

In an implementation, the LBARs and the solar cell base may be composedof the same elements, but with the composition of the LBARs giving alower bandgap than the solar cell base. In an implementation, the SCRsand the solar cell base may be composed of the same elements, but withthe composition of the SCRs giving a material lattice constant whichresults in a strain-balanced or zero net strain array. In animplementation, the LBARs and the SCRs may be composed of the sameelements, but with the composition of the LBARs giving a lower bandgapthan the solar cell base, and the composition of the SCRs giving amaterial lattice constant which results in a strain-balanced or zero netstrain array when combined with the LBARs within the solar cell. In animplementation, the LBARs, the SCRs, and the solar cell base may becomposed of the same elements, but with the composition of the LBARsgiving a lower bandgap than the solar cell base, and the composition ofthe SCRs giving a material lattice constant which results in astrain-balanced or zero net strain array when combined with the LBARswithin the solar cell.

In an implementation, the LBARs and the SCRs may be composed ofdifferent elements, with the composition of the LBARs giving a lowerbandgap than the solar cell base, and the composition of the SCRs givinga material lattice constant which results in a strain-balanced or zeronet strain array when combined with the LBARs within the solar cell. Inan implementation, the LBARs, the SCRs, and the solar cell orphotovoltaic cell base may be composed of different elements, with thecomposition of the LBARs giving a lower bandgap than the solar cellbase, and the composition of the SCRs giving a material lattice constantwhich results in a strain-balanced or zero net strain array whencombined with the LBARs within the solar cell.

In an implementation, the SCRs may be positioned between each pair ofLBARs. In an implementation, the LBARs may be positioned between eachpair of SCRs.

In an implementation the solar cells with LBARs and/or SCRs incorporatedare subcells within a multijunction cell, for which the solar spectrumutilization is improved and the energy conversion efficiency is mademore efficient through the change in spectral response resulting fromincorporation of the LBARs and/or SCRs.

The present disclosure further describes a high-efficiency multijunction(MJ) photovoltaic (PV) cell that may be used with, for example, aterrestrial concentrator photovoltaic (CPV) electricity generationsystem, or a satellite for use in space. One example of a multijunctionPV cell is a 3-junction GaInP/Ga(In)As/Ge cell, but the multijunctioncell structure is by no means limited to a 3-junction structure, and infact a wide variety of multijunction cell configurations are envisionedin the present disclosure.

The multijunction solar cell may have 2, 3, 4, 5, 6, 7, 8, or 9 or moresubcells, also referred to a p-n junctions or simply “junctions,” inorder to convert the incident spectrum, such as the concentrated orunconcentrated solar spectrum, more efficiently. Multijunction solarcells and the subcells of the multijunction cell are both types of solarcell, and are referred to collectively as solar cells in the followingtext. The multijunction solar cells and the subcells of themultijunction cell may have a variety of structures and configurations,as described below.

In one implementation, one or more of the solar cells described hereinmay be a lattice-mismatched or metamorphic solar cell with latticeconstant different from that of a growth substrate. In oneimplementation, one or more of the solar cells described herein may be alattice-matched solar cell with lattice constant approximately the sameas that of a growth substrate. In one implementation, two or more of thesubcells in the multijunction cell may be lattice-matched to each other.In one implementation, all of the subcells in the multijunction cell maybe lattice-matched to each other. In one implementation, all of thesubcells in a multijunction cell may be lattice-matched to a growthsubstrate. In another implementation, one or more lattice-matchedsubcells may be integrated with one or more lattice-mismatched ormetamorphic cells in the multijunction cell. In another implementation,all the subcells in the multijunction cell may be lattice-mismatched ormetamorphic.

In one implementation, one or more of the solar cells described hereinmay be grown in an inverted configuration, where an inverted growthconfiguration is defined such that the layers of the solar cell designedto be toward the sun or other light source during solar cell operation(layers toward the sunward surface) are grown first, followed by therest of the solar cell layers, finishing with the layers of the solarcell designed to be away from the sun or other light source during solarcell operation (layers away from the sunward surface). In oneimplementation, one or more of the solar cells described herein may begrown in an upright configuration, where an upright growth configurationis defined such that the layers of the solar cell designed to be awayfrom the sun or other light source during solar cell operation (layersaway from the sunward surface) are grown first, followed by the rest ofthe solar cell layers, finishing with the layers of the solar celldesigned to be toward the sun or other light source during solar celloperation (layers toward the sunward surface). Such an uprightconfiguration is in contrast to an inverted configuration. In oneimplementation, the multijunction cell may comprise one or more subcellsgrown with an upright configuration, and one or more subcells grown withan inverted configuration.

In one implementation, one or more of the solar cells described abovemay be an inverted metamorphic cell, with an inverted growthconfiguration as defined above, and with a base or primaryphotogeneration or absorber layer having a material lattice constantthat is different from that of a growth substrate. In oneimplementation, one or more of the solar cells described above may be aninverted lattice-matched cell, with an inverted growth configuration asdefined above, and with a base or primary photogeneration or absorberlayer having a material lattice constant that is approximately the sameas that of a growth substrate.

In one implementation, solar cells described herein may havesemiconductor layers that are grown by an epitaxial semiconductor growthprocess, such as metal-organic vapor-phase epitaxy (MOVPE), molecularbeam epitaxy (MBE), metal-organic molecular beam epitaxy (MOMBE), liquidphase epitaxy (LPE), vapor phase epitaxy (VPE), and other growthmethods. In one implementation, one or more layers of the solar cellsdescribed herein may be formed by physical vapor deposition (PVD),chemical vapor deposition (CVD), plasma-enhanced chemical vapordeposition (PECVD), low-pressure chemical vapor deposition (LPCVD),evaporation, sputtering, screen printing, spray coating, spin coating,doctor blade application, surface wetting by a liquid, electrolessplating, electroplating, photolithography, shadow mask deposition,isotropic etching, composition-selective etching, orientation-selectiveetching, directionally-selective etching, photopatterning of a resistlayer followed by etching a layer of the solar cell, photopatterning ofa first layer of the solar cell followed by etching a second layer ofthe solar cell using the patterned first layer as an etch mask in aself-aligned process, photopatterning of a resist layer followed bydeposition of a material and dissolution of the resist layer to removepart of the deposited material in a liftoff process, or photopatterningof a photosensitive layer that is a layer of the solar cell.

In addition, in some implementations comprising multijunction solarcells, the subcells may be mechanically, optically, and electricallyintegrated together to form the multijunction solar cell in a variety ofways. In one implementation, for example, the subcells may be integratedor joined together by epitaxially growing the constituent semiconductorlayers of one subcell, followed by growth of an interconnection seriesof layers that provides optical and electrical coupling betweensubcells, such as a tunnel junction, followed by growth of theconstituent semiconductor layers of an additional subcell to beintegrated with the subcell or subcells already grown, and repeating asmany times as needed to form and interconnect all the subcells in themultijunction cell, or in the portion of the multijunction cell that isbeing grown.

Subcells or combinations of subcells grown on separate growth substratesmay also be joined together using a broad class of structures and modesof subcell integration referred to as bonding structures. Such amultijunction solar cell can be referred to as a bonded multijunctionsolar cell. In one implementation, one or more interfaces betweensubcells in the multijunction solar cell is a bonding structure, chosenfrom the following group of bonding structures and subcell integrationmodes:

1) transparent adhesive bonding combined with a patterned metalconductor or interconnect;2) transparent adhesive bonding combined with a patterned or unpatternedtransparent conductor;3) transparent inorganic layer bonding, using materials such as but notlimited to oxides or nitrides, combined with a patterned metalconductor;4) transparent inorganic layer bonding, using materials such as but notlimited to such as oxides or nitrides, combined with a patterned orunpatterned transparent conductor;5) bonding of transparent conductive coatings such as but not limited tozinc oxide or indium tin oxide;6) direct semiconductor-to-semiconductor bonding, or semiconductorbonding technology; or7) other subcell integration configuration.

The transparent adhesive layers in 1) and 2) and the transparentinorganic layers in 3) and 4) may be relatively compliant to surfacenonuniformities, softened, or liquid under the bonding conditions, ormay be relatively rigid in their mechanical properties. Likewise, themetal or transparent conductors imbedded in a transparent medium may berelatively compliant, softened, or molten under the bonding conditions,or may be relatively rigid. The metal or non-transparent conductors inmethods 1) and 3) can be patterned or distributed such that they coveronly a small fraction, typically between 0.5% and 10% of the cellsurface, so that the bonded interface as a whole remains largelytransparent, allowing light of the desired wavelengths, e.g. sunlightthat is transmitted by the upper subcells, to be efficiently transmittedto the subcells beneath the bonded interface. The bonding methods in 1),2), 3), and 4) are referred to generally here as mechanical stacking ofsubcells. The bonding methods in 5) and 6) may also be thought of asmechanical stacking of subcells.

Moreover, anti-reflection (AR) coatings and/or transparent conductivecoatings (TCCs) may be combined with the transparent layer bondingmethods in 1), 2), 3), and 4). The transparent conductive coatingsemployed may also serve as anti-reflection coatings.

In some implementations, subcells may be grown on separate growthsubstrates and integrated or bonded together using a transparentadhesive material, such as but not limited to a silicone polymer,combined with a patterned or unpatterned transparent conductor, such asbut not limited to transparent conductive oxide particles imbedded in atransparent matrix, or patterned metal conductor to provide electricalconnection between the bonded subcells. In one implementation, subcellsmay be grown on separate growth substrates and integrated or bondedtogether by forming a bond between transparent inorganic layers on eachsurface, such as but not limited to silicon dioxide or silicon nitride,combined with a patterned or unpatterned transparent conductor, orpatterned metal conductor to provide electrical connection between thebonded subcells. In one implementation, subcells may be grown onseparate growth substrates and integrated or bonded together by forminga bond between transparent conductive coatings (TCCs) on each surface,such as but not limited to zinc oxide or indium tin oxide, which allowslight to be transmitted through the bonding layers and provideselectrical connection between the bonded subcells, and may optionally becombined with an additional patterned or unpatterned transparentconductor or patterned metal conductor to enhance electrical connectionbetween subcells. In one implementation, subcells may be grown onseparate growth substrates and integrated or bonded together by forminga semiconductor-to-semiconductor bond, which can be an atomically abruptdirect interface between dissimilar semiconductors that is bothoptically transparent and electrically conductive. This bonding methodinvolving a direct bond from one semiconductor to another is generallyreferred to as semiconductor-to-semiconductor bonding, directsemiconductor bonding, or as semiconductor bonding technology (SBT). Thesemiconductor-to-semiconductor bond is typically atomically abrupt, withthe transition from one semiconductor to the other taking place within 1to 10 monolayers of the crystal lattice.

In some implementations, subcells may be integrated or bonded togetherto form a multijunction cell using a combination of the abovestructures, such as a first combination of multiple subcells growninverted in a monolithic multijunction stack on a first substrate, andbonded to a second combination of multiple subcells grown upright in amonolithic multijunction stack on a second substrate using adhesivebonding technology, transparent inorganic layer bonding technology,transparent conductive coating bonding technology, or semiconductorbonding technology (SBT), followed by removal of the substrate on thesunward surface to allow light to enter the sunward cell surface, andoptionally, removal of the substrate on the side away from the lightsource, to confer lighter weight, flexibility, better thermal transfer,lower cost through substrate reuse, or other beneficial quality to theintegrated cell.

In one implementation, one or more of the solar cells described hereinmay be incorporated in a stack of subcells forming a multijunction solarcell that is grown:

1) on a single side of a growth substrate;2) on both sides of a growth substrate;3) in a single growth run;4) in two or more growth runs;5) in a single growth run on a single side of the growth substrate;6) in a single growth run on both sides of the growth substrate;7) in two or more growth runs on a single side of the growth substrate;or8) in two or more growth runs on both sides of the growth substrate.

In some implementations, a subcell in the a mechanically-stacked,TCC-bonded, or semiconductor-bonded multijunction solar cell describedherein may have a different material lattice constant than some othersubcells in the multijunction stack, where the subcells are integratedby lattice-mismatched growth, metamorphic growth with a graded bufferlayer, mechanical stacking, transparent conductive coating (TCC)bonding, or semiconductor bonding. Such a subcell in themechanically-stacked, TCC-bonded, or semiconductor-bonded multijunctionsolar cell may also have approximately the same material latticeconstant as other subcells in the multijunction stack, where thesubcells are integrated by lattice-matched growth, mechanical stacking,TCC bonding, or semiconductor bonding. Likewise, a subcell in themechanically-stacked, TCC-bonded, or semiconductor-bonded multijunctionsolar cell may have been grown on a different growth substrate than someother subcells in the multijunction stack. Such a subcell may also havebeen grown on the same growth substrate as other subcells in themultijunction stack, where some subsets of subcells in the multijunctionstack may be grown on the same growth substrate with an upright growthconfiguration, with an inverted growth configuration, or both.

In one implementation, the subcells may be electrically interconnectedin series using heavily-doped p+/n+ tunnel junctions grown between thesubcells. In one implementation, the subcells may be electricallyinterconnected in series using metal or transparent conductors withpartial coverage fraction imbedded in a transparent medium between thesubcells. In one implementation, the subcells may be electricallyinterconnected in series through a bond between one or more transparentconductive coating (TCC) layers, semiconductor layers, or other subcelllayers. In another implementation, the subcell electrical terminals maybe accessed independently of other subcell terminals at the side,through the front, or through the back of the cells, to combine theelectrical output of individual subcells, in a way which need not havethe current matching constraints among subcells typical ofseries-connected multijunction cell configurations, and can thereforehave higher efficiencies and have greater tolerance of non-ideal bandgapcombinations in the multijunction cell.

Although the example multijunction solar cells above have been describedwith specific structures achieved by specific modes of integrating thesubcells into a multijunction cell, the modes of interconnection may beapplied to multijunction solar cells with any number of subcells, andbetween any combination of subcells, even if different than the examplesshown, and may include other modes of interconnection, and still beincluded within the present disclosure. Examples of other modes ofinterconnection include optical interconnection among discrete orphysically separated single junction or multijunction solar cells in aspectral splitting receiver system, e.g., the combination of a3-junction (Al)GaInP/(Al)Ga(In)As/Ge solar cell in combination with asilicon solar cell in a spectral splitting optical system, or thecombination a 4-junction (Al)GaInP/AlGa(In)As/(Al)Ga(In)As/Ge solar cellin combination with a silicon solar cell in a spectral splitting opticalsystem.

In addition, LBARs and SCRs described herein may also be placed in asolar cell with an ordered GaInP (o-GaInP) base, a disordered GaInP(d-GaInP) base, an ordered AlGaInP base, or a disordered AlGaInP base.The terms ordered and disordered refer to the positions group-III atomsin the periodic table (Al, Ga, In, etc.) on the group-III sublattice.Whether the semiconductor is ordered or disordered in this sense, thecrystal lattice can have, and ideally does have, a perfectly periodiccrystal structure. At the same semiconductor composition, i.e., ratio ofGa to In, disordered GaInP has a bandgap that is on the order of 100 meVhigher than GaInP with partial group-III ordering that is readilyachievable in practice (referred to here as ordered GaInP, although thegroup-III ordering may not be complete, i.e., the ordering parameter maybe less than unity), near the GaAs lattice constant. This effect ofgroup-III sublattice disordering on bandgap has been confirmed inmetamorphic GaInP solar cells with higher indium composition and latticeconstant as well as for GaInP lattice matched to Ge or GaAs, and alsooccurs in AlGaInP. The bandgap change with disordering diminishes as theGaInP composition approaches that of InP, but for most metamorphic GaInPcompositions of interest for solar cells, e.g. from about 0 to 20% Incomposition, the bandgap change with disordering is still quitesignificant.

The extension of the long-wavelength response of the solar cell due tothe array of LBARs, to increase the photogenerated current density ofthat solar cell, which may be a subcell in a multijunction solar cell,can be used in general to optimize the current balance among subcells inthe multijunction solar cell for a typical range of incident spectra andrange of solar cell temperatures, thus increasing the efficiency of themultijunction solar cell, improving its ease of manufacturability,and/or reducing its cost of manufacture, according to one or moreimplementations of the present disclosure. The principle of using0-dimensional (0D), 1-dimensional (1D), or 2-dimensional (2D) LBARs tooptimize the current balance among subcells in a multijunction cell inthe present disclosure, with the LBARs placed in the quasi-neutralregions, or other regions or combinations of other regions of thedevice, in the top subcell, bottom subcell, or other subcells of themultijunction solar cell or other optoelectronic device, to improve itsefficiency or performance in a way that could not easily be accomplishedotherwise, can be applied to 2-junction solar cells, 3-junction cells,4-junction cells, 5-junction cells, 6-junction cells, and solar cellswith 7 or more junctions, and to other optoelectronic devices.

Accordingly, a semiconductor structure is disclosed that, in someimplementations, achieves higher minority-carrier lifetime,minority-carrier mobility, minority-carrier diffusion length,majority-carrier mobility, majority-carrier conductance—referred tocollectively as carrier transport properties—higher photogeneratedcurrent density, and/or higher collected current density in response tolight, due to one or more low-bandgap absorber regions (LBARs), that mayalso function as improved transport (IT) layers positioned in thequasi-neutral region of the emitter of a solar cell.

A semiconductor structure is also disclosed that in someimplementations, achieves higher carrier transport properties, higherphotogenerated current density, and/or higher collected current densityin response to light, in particular light at short wavelengths, due toan aluminum-free region that forms part or all of the emitter of a solarcell having an aluminum-containing base, where the aluminum-free regionin the emitter forms an IT layer, and may or may not also form alow-bandgap absorber region (LBAR).

A semiconductor structure is also disclosed that, in someimplementations, achieves higher carrier transport properties, higherphotogenerated current density, and/or higher collected current densityin response to light, in particular light at short wavelengths, due to alow-aluminum-content region that forms part or all of the emitter of asolar cell having an aluminum-containing base, wherelow-aluminum-content is defined as being lower than the aluminum contentof the base, where the low-aluminum-content region in the emitter formsan IT layer, and may or may not also form a low-bandgap absorber region(LBAR).

A semiconductor structure is also disclosed that, in someimplementations, achieves higher carrier transport properties, higherphotogenerated current density, and/or higher collected current densityin response to light, due to a nitrogen-free region, foaming an ITlayer, that forms part or all of the emitter of a solar cell having annitrogen-containing base, such as a solar cell comprising a dilutenitride GaInNAs(Sb) base layer.

A semiconductor structure is also disclosed that, in someimplementations, achieves higher carrier transport properties, higherphotogenerated current density, and/or higher collected current densityin response to light, due to one or more low-bandgap absorber regions(LBARs) and/or IT layers positioned in parts of the solar cell otherthan the emitter, such as the quasi-neutral region of the base or in thespace-charge region of a solar cell.

A semiconductor structure is also disclosed with a primaryphotogeneration layer comprising an integer n chemical elements, withhigher carrier transport properties, higher photogenerated currentdensity, and/or higher collected current density in response to light,due to one or more layers with similar composition as the primaryphotogeneration layer, but with reduced content of one or more of the nchemical elements as compared to the primary photogeneration layer,forming one or more IT layers and/or one or more LBAR layers.

A semiconductor structure is also disclosed with a primaryphotogeneration layer comprising an integer n chemical elements, withhigher carrier transport properties, higher photogenerated currentdensity, and/or higher collected current density in response to light,due to one or more layers with similar composition as the primaryphotogeneration layer, but with zero content of one or more of the nchemical elements, forming one or more IT layers and/or one or more LBARlayers.

A semiconductor structure is also disclosed with a primaryphotogeneration layer comprising a binary (n=2), ternary (n=3),quaternary (n=4), pentanary (n=5) semiconductor, or a semiconductor with6 or more (n≧6) chemical elements in its composition, with higherminority-carrier lifetime, minority-carrier mobility, minority-carrierdiffusion length, majority-carrier mobility, majority-carrierconductance, higher photogenerated current density, and/or highercollected current density in response to light, due to one or morelayers with similar composition as the primary photogeneration layer,but with reduced or eliminated content of one or more of the n=2, n=3,n=4, n=5, or n≧6 chemical elements as compared to the primaryphotogeneration layer, forming one or more IT layers and/or one or moreLBAR layers.

Examples of semiconductor materials for solar cells and othersemiconductor devices having an emitter that comprises an LBAR and/or anIT layer include, but are not limited to, the following combinations ofemitter and base materials:

GaInP emitter/AlGaInP base;low-Al AlGaInP emitter/AlGaInP base;GaAs or low-Al AlGaAs emitter/AlGaAs base;GaAs or low-Al AlGaAs emitter/AlGaInAs base;GaInAs or low-Al AlGaInAs emitter/AlGaAs base;GaInAs or low-Al AlGaInAs emitter/AlGaInAs base;GaAs or low-P GaPAs emitter/GaInPAs base;GaInAs or low-P GaInPAs emitter/GaInPAs base;GaInAs or low-Al AlGaInPAs emitter/AlGaInPAs base;GaInAs or low-P AlGaInPAs emitter/AlGaInPAs base;GaAsSb or low-Al AlGaAsSb emitter/AlGaAsSb base;GaAsSb or low-Al AlGaInAsSb emitter/AlGaInAsSb base;GaInAs or low-Al AlGaInPAsSb emitter/AlGaInPAsSb base;GaInAs or low-P AlGaInPAsSb emitter/AlGaInPAsSb base;GaInP emitter with high group-III sublattice ordering/GaInP base withlow or zero group-III sublattice ordering;AlGaInP emitter with high group-III sublattice ordering/AlGaInP basewith low or zero group-III sublattice ordering;GaInAs emitter with high group-III sublattice ordering/GaInAs base withlow or zero group-III sublattice ordering;AlGaInAs emitter with high group-III sublattice ordering/AlGaInAs basewith low or zero group-III sublattice ordering;GaAs emitter/GaInNAs or GaInNAsSb base;GaInAs emitter/GaInNAs or GaInNAsSb base;low-N GaInNAs emitter/GaInNAs or GaInNAsSb base;low-N GaInNAsSb emitter/GaInNAs or GaInNAsSb base;GaInP emitter/GaInNAs or GaInNAsSb base;AlInP emitter/GaInNAs or GaInNAsSb base;GaAsSb emitter/GaInNAs or GaInNAsSb base;GaInAsSb emitter/GaInNAs or GaInNAsSb base;GaAs emitter/AlGaInNAs or AlGaInNAsSb base;GaInAs emitter/AlGaInNAs or AlGaInNAsSb base;low-Al AlGaInNAs emitter/AlGaInNAs or AlGaInNAsSb base;low-Al AlGaInNAsSb emitter/AlGaInNAs or AlGaInNAsSb base;GaN emitter/AlGaN base;GaInN emitter/AlGaInN base;low-Al AlGaInN emitter/AlGaInN base;InN emitter/GaInN base.

Some exemplary implementations will now be described with reference tothe figures. FIG. 1 illustrates an example of a solar cell according toan implementation of the disclosure, that may form a subcell within amultijunction or single junction solar cell, with one or more layerswith improved charge carrier transport properties in the emitter, andforming one or more low-bandgap absorber regions (LBARs) in the emitter.The improved transport (IT) and LBAR layer(s) may be partly in theemitter quasi-neutral region and partly in the emitter space-chargeregion, or may be entirely in the emitter quasi-neutral region, or maybe entirely in the emitter space-charge region. The example in the FIG.1 shows an AlGaInP-base subcell with Al-free GaInP forming the entireemitter, where the improved charge carrier transport properties andlower bandgap compared to the base are due to the absence of aluminum(Al) in the emitter composition. The GaInP emitter and the AlGaInP basemay be either ordered or disordered on the group-III sublattice. Therelationships between bandgap and lattice constant of the LBAR layer orlayers and other device layers are diagrammed on the left of FIG. 1, asthey are in many of the following figures.

FIG. 2 illustrates an example of a solar cell according to animplementation of the disclosure, that may form a subcell within amultijunction or single junction solar cell, with an AlGaInP base, andwith an Al-free GaInP layer within the emitter having improved chargecarrier transport properties and forming an LBAR in the emitter, wherethe Al-free GaInP layer is partly in the emitter quasi-neutral regionand partly in the emitter space-charge region, is in contact with thep-type AlGaInP base, and is separated from the window layer by an n-typeAlGaInP part of the emitter. The LBAR and improved transport layer maybe n-type or intrinsic in doping type. The LBAR thickness may range from0% to 100% of the total emitter thickness, and may be varied to affectcurrent balance in a multijunction cell.

FIG. 3 illustrates an example of a solar cell according to animplementation of the disclosure, that may form a subcell within amultijunction or single junction solar cell, with an AlGaInP base, andwith an Al-free GaInP layer within the emitter having improved chargecarrier transport properties and forming an LBAR in the emitter, wherethe Al-free GaInP layer is partly in the emitter quasi-neutral regionand partly in the emitter space-charge region, is separated from thebase layer by an n-type or intrinsic AlGaInP part of the emitter, and isseparated from the window layer by an n-type AlGaInP part of theemitter.

FIG. 4 illustrates an example of a subcell within a solar cell, with anAlGaInP base, and with an Al-free GaInP layer within the emitter havingimproved charge carrier transport properties and forming an LBAR in theemitter, where the Al-free GaInP layer is entirely in the emitterquasi-neutral region, is in contact with the n-type AlInP window, and isseparated from the base layer by an n-type or intrinsic AlGaInP part ofthe emitter.

FIG. 5 illustrates an example of a subcell within a solar cell, with anAlGaInP base, and with an Al-free GaInP layer within the emitter havingimproved charge carrier transport properties and forming an LBAR in theemitter, where the Al-free GaInP layer is entirely in the emitterquasi-neutral region, is separated from the p-type AlGaInP base by ann-type or intrinsic AlGaInP part of the emitter, and is separated fromthe window layer by an n-type AlGaInP part of the emitter.

FIG. 6 illustrates an example of a subcell within a solar cell, with anAl-free GaInP layer forming the entire n-type emitter, having improvedcharge carrier transport properties in the emitter and forming alow-bandgap absorber region (LBAR), and with a p-type AlGaInP base withhigher bandgap with respect to the emitter and/or reduced thickness,such that the emitter layer is a major photoabsorbing region of thesolar cell, such that 30-100%, and preferably 50-100%, of thephotogeneration in the solar cell comes from the emitter layer. TheAl-free GaInP emitter is partly in the solar cell quasi-neutral region,and partly in the solar cell space-charge region. The LBAR thickness mayrange from 5% to 100% of the total solar cell photoabsorber thickness,and may be varied to affect current balance in a multijunction cell.

FIG. 7 illustrates an example of a subcell within a solar cell, with anAl-free GaInP layer forming the entire n-type emitter, having improvedcharge carrier transport properties in the emitter and forming alow-bandgap absorber region (LBAR), and forming a p-n junction betweenthe emitter layer and a back surface field (BSF) layer with higherbandgap than the emitter layer, where there is no base layer (zerothickness base layer) with the same or lower bandgap as the emitterlayer but where the BSF layer may also be thought of as having the dualrole of a base layer since it forms a p-n junction with the emitter,such that the emitter layer is a major photoabsorbing region of thesolar cell, such that 50-100%, and preferably 90-100%, of thephotogeneration in the solar cell comes from the emitter layer. TheAl-free GaInP emitter is partly in the solar cell quasi-neutral region,and partly in the solar cell space-charge region.

FIG. 8 illustrates an example of a subcell within a solar cell, withemitter layers having bandgaps lower than the p-type base, p-type BSF,and n-type window layers, having improved charge carrier transportproperties in the emitter and forming one or more low-bandgap absorberregions (1st level LBARs) in the emitter, and further having a lowerbandgap absorber region (2nd level LBAR) with lower bandgap than the 1stlevel LBARs. The 2nd level LBAR may be partly in the space-charge regionand partly in the quasi-neutral region, entirely within the space-chargeregion, or entirely within the quasi-neutral region. In the exampleshown in FIG. 8, AlGaInP with lower Al-content than in the base formsthe 1st level LBAR layers, and Al-free GaInP forms the 2nd level LBAR,which is partly in the space-charge region and partly in thequasi-neutral region.

FIG. 9 illustrates an example of a subcell within a solar cell, with oneor more layers with improved charge carrier transport properties in thebase, and forming one or more low-bandgap absorber regions (LBARs) inthe base. The improved transport (IT) and LBAR layer(s) may be partly inthe base quasi-neutral region and partly in the base space-chargeregion, or may be entirely in the base quasi-neutral region, or may beentirely in the base space-charge region. The example in FIG. 9 shows asubcell with an AlGaInP emitter, and with an Al-free GaInP layer withinthe base having improved charge carrier transport properties and formingan LBAR in the base, where the Al-free GaInP layer is partly in the basequasi-neutral region and partly in the base space-charge region, is incontact with the n-type AlGaInP emitter, and is separated from the backsurface field (BSF) layer by a p-type AlGaInP part of the base. The LBARthickness may range from 0% to 100% of the total base thickness, and maybe varied to affect current balance in a multijunction cell.

FIG. 10 illustrates an example of a subcell within a solar cell, with anAlGaInP emitter, and with an Al-free GaInP layer within the base havingimproved charge carrier transport properties and forming an LBAR in thebase, where the Al-free GaInP layer is partly in the base quasi-neutralregion and partly in the base space-charge region, is separated from theemitter layer by a p-type or intrinsic AlGaInP part of the base, and isseparated from the back surface field (BSF) layer by a p-type AlGaInPpart of the base.

FIG. 11 illustrates an example of a subcell within a solar cell, with anAlGaInP emitter, and with an Al-free GaInP layer within the base havingimproved charge carrier transport properties and forming an LBAR in thebase, where the Al-free GaInP layer is entirely in the basequasi-neutral region, is separated from the emitter layer by a p-type orintrinsic AlGaInP part of the base, and is in contact with the backsurface field (BSF) layer.

FIG. 12 illustrates an example of a subcell within a solar cell, with anAlGaInP emitter, and with an Al-free GaInP layer within the base havingimproved charge carrier transport properties and forming an LBAR in thebase, where the Al-free GaInP layer is entirely in the basequasi-neutral region, is separated from the emitter layer by a p-type orintrinsic AlGaInP part of the base, and is separated from the backsurface field (BSF) layer by a p-type AlGaInP part of the base.

FIG. 13 illustrates an example of a subcell within a solar cell, with animproved charge carrier transport layer and/or low bandgap absorberregion (LBAR) forming the entire p-type base, and having an emitter withhigher bandgap with respect to the base and/or reduced thickness, suchthat the base layer is a major photoabsorbing region of the solar cell,such that 30-100%, and preferably 50-100%, of the photogeneration in thesolar cell comes from the base layer. The improved transport (IT) and/orLBAR layer in the base is partly in the solar cell quasi-neutral region,and partly in the solar cell space-charge region. The IT and/or LBARthickness may range from 5% to 100% of the total solar cellphotoabsorber thickness, and may be varied to affect current balance ina multijunction cell. In the example shown in FIG. 13, the IT/LBAR baselayer and the emitter layer are composed of Al-free GaInP and AlGaInP,respectively, but they may be composed of other materials as well, suchas an ordered GaInP base and disordered (Al)GaInP emitter, or anunstrained GaInP base and a tensile-strained pseudomorphic AlInPwindow/emitter with higher Al content and higher bandgap than for anunstrained lattice matched AlInP layer.

FIG. 14 illustrates an example of a subcell within a solar cell, with animproved charge carrier transport layer and/or low bandgap absorberregion (LBAR) forming the entire p-type base, and forming a p-n junctionbetween the base layer and a window layer with higher bandgap than thebase layer, where there is no emitter layer (zero thickness emitterlayer) with the same or lower bandgap as the base layer but where thewindow layer may also be thought of as having the dual role of anemitter layer since it forms a p-n junction with the base, such that thebase layer is a major photoabsorbing region of the solar cell, such that50-100%, and preferably 90-100%, of the photogeneration in the solarcell comes from the base layer. The improved transport (IT) and/or LBARlayer in the base is partly in the solar cell quasi-neutral region, andpartly in the solar cell space-charge region. In the example shown inFIG. 14, the IT/LBAR base layer and the window/emitter layer arecomposed of Al-free GaInP and AlInP, respectively, but they may becomposed of other materials as well, such as an ordered GaInP base anddisordered (Al)(Ga)InP window/emitter, or an unstrained GaInP base and atensile-strained pseudomorphic AlInP window/emitter with higher Alcontent and higher bandgap than for an unstrained lattice matched AlInPlayer.

FIG. 15 illustrates an example of a subcell within a solar cell, withbase layers having bandgaps lower than the n-type emitter, n-typewindow, and p-type BSF layers, having improved charge carrier transportproperties in the base and forming one or more low-bandgap absorberregions (1st level LBARs) in the base, and further having a lowerbandgap absorber region (2nd level LBAR) with lower bandgap than the 1stlevel LBARs. The 2nd level LBAR may be partly in the space-charge regionand partly in the quasi-neutral region, entirely within the space-chargeregion, or entirely within the quasi-neutral region. In the exampleshown in FIG. 15, AlGaInP with lower Al-content than in the emitterforms the 1st level LBAR layers, and Al-free GaInP forms the 2nd levelLBAR, which is partly in the space-charge region and partly in thequasi-neutral region.

FIG. 16 illustrates an example of a subcell within a solar cell, with anAlGaInP base and a low-Al-content AlGaInP emitter with respect to the Alcomposition of the base, with improved charge carrier transportproperties in the emitter, and forming an LBAR in the emitter. Any ofthe examples disclosed herein with an improved transport (IT) and/orLBAR layer formed from Al-free GaInP may instead have a low-Al-contentAlGaInP IT and/or LBAR layer, with low Al composition with respect toother photogeneration layers in the solar cell.

FIG. 17 illustrates an example of a subcell within a solar cell, with aGaInP or AlGaInP base that is largely or completely disordered on thegroup-III sublattice (disordered (Al)GaInP or d-(Al)GaInP), and anemitter composed of GaInP or AlGaInP with some degree of ordering on thegroup-III sublattice (ordered (Al)GaInP or o-(Al)GaInP), resulting inlower bandgap in the ordered material with respect to the disorderedmaterial, forming a low bandgap absorber region (LBAR) in the emitter,and/or an improved transport (IT) layer or layers in the emitter orbase, due to the degree of order or disorder on the group-IIIsublattice.

FIG. 18 illustrates an example of a subcell within a solar cell, with adisordered (Al)GaInP (meaning GaInP or AlGaInP) emitter, and with anordered GaInP or AlGaInP layer within the quasi-neutral region of thebase, having lower bandgap than for disordered material and forming anLBAR, and/or an improved transport (IT) layer or layers in the emitteror base, due to the degree of order or disorder on the group-IIIsublattice.

FIG. 19 illustrates an example of a subcell within a solar cell, with adisordered (Al)GaInP (meaning GaInP or AlGaInP) emitter, and with anordered GaInP or AlGaInP layer within the space-charge region of thebase, having lower bandgap than for disordered material and forming anLBAR, and/or an improved transport (IT) layer or layers in the emitteror base, due to the degree of order or disorder on the group-IIIsublattice.

FIG. 20 illustrates an example of a subcell within a solar cell, with adisordered (Al)GaInP (meaning GaInP or AlGaInP) emitter, and with anordered GaInP or AlGaInP layer partly within the quasi-neutral region ofthe base and partly within the space-charge region of the base, havinglower bandgap than for disordered material and forming an LBAR, and/oran improved transport (IT) layer or layers in the emitter or base, dueto the degree of order or disorder on the group-III sublattice.

FIG. 21 illustrates an example of a subcell within a solar cell, with ap-type AlGaInP base, and with an n-type Al-free GaInP emitter faultingan LBAR and improved transport (IT) layer in the emitter, in combinationwith a high-Al-content, pseudomorphic, AlInP window in tensile strainwith respect to the emitter to achieve higher bandgap and greatertransparency of the window. Here high-Al-content in the AlInP emittermeans that the Al content is higher than the AlInP composition with thesame material lattice constant as the AlGaInP base, and the GaInPemitter is lattice-matched and unstrained with respect to the AlGaInPsolar cell base.

FIG. 22 illustrates an example of a subcell within a solar cell, with ap-type AlGaInP base, and with an n-type, pseudomorphic, Al-free GaInPemitter in compressive strain with respect to the AlGaInP base, formingan LBAR and improved transport (IT) layer in the emitter, in combinationwith a high-Al-content, pseudomorphic, AlInP window in tensile strainwith respect to the emitter to achieve higher bandgap and greatertransparency of the window. Here high-Al-content in the AlInP emittermeans that the Al content is higher than the AlInP composition with thesame material lattice constant as the AlGaInP base, and thecompressively-strained GaInP emitter balances the tensile strain in theAlInP window.

FIG. 23 illustrates an example of a subcell within a solar cell, with ap-type AlGaInP base, and with an n-type, pseudomorphic, AlGaInP emitterin compressive strain with respect to the AlGaInP base and with Alcontent in the emitter such that the emitter bandgap may be less than,the same as, or greater than the AlGaInP base, in combination with ahigh-Al-content, pseudomorphic, AlInP window in tensile strain withrespect to the emitter to achieve higher bandgap and greatertransparency of the window. Here high-Al-content in the AlInP emittermeans that the Al content is higher than the AlInP composition with thesame material lattice constant as the AlGaInP base, and thecompressively-strained AlGaInP emitter balances the tensile strain inthe AlInP window.

FIG. 24 illustrates an example of a subcell within a solar cell, with aGaInP or AlGaInP base and with LBARs and strain compensation regions(SCRs) which may take the form of layers. The LBARs and straincompensation regions may be partly in the space-charge region and partlyin the quasi-neutral region of the base as shown, or may be partly inthe space-charge region and partly in the quasi-neutral region of theemitter, or may be partly in the space-charge region and partly in thequasi-neutral regions of both the emitter and the base, or may beentirely in the quasi-neutral region of the base, or may be entirely inthe quasi-neutral region of the emitter, or may be entirely in thespace-charge region. Any of the examples disclosed herein with animproved transport (IT) and/or LBAR layer in the cell structure mayinstead use a combination of LBARs and strain compensation regions.

FIG. 25 illustrates an example of a subcell within a solar cell, with ann-type dilute nitride GaInNAs(Sb) emitter layer forming an LBAR in theemitter due to the reduction in bandgap due to nitrogen incorporation,and a GaAs, GaInAs, or low-nitrogen-content GaInNAs(Sb) base, with lowerN content and higher bandgap with respect to the emitter, forming animproved transport (IT) layer in the base, due to the absence or lowerconcentration of N in the base layers.

FIG. 26 illustrates an example of a subcell within a solar cell, with ap-type dilute nitride GaInNAs(Sb) base layer forming an LBAR in the basedue to the reduction in bandgap due to nitrogen incorporation, and aGaAs, GaInAs, or low-nitrogen-content GaInNAs(Sb) emitter, with lower Ncontent and higher bandgap with respect to the base, forming an improvedtransport (IT) layer in the emitter, due to the absence or lowerconcentration of N in the emitter layers.

FIG. 27 illustrates an example of a subcell within a solar cell, with athick, n-type dilute nitride GaInNAs(Sb) emitter layer, forming an LBARdue to the reduction in bandgap due to nitrogen incorporation, such thatthe emitter layer is a major photoabsorbing region of the solar cell,such that 30-100%, and preferably 50-100%, of the photogeneration in thesolar cell comes from the emitter layer, and with an optional GaAs,GaInAs, or low-nitrogen-content GaInNAs(Sb) base, with lower N contentand higher bandgap with respect to the emitter.

FIG. 28 illustrates an example of a subcell within a solar cell, with ap-type dilute nitride GaInNAs(Sb) layer within the base, forming an LBARin the base due to the reduction in bandgap due to nitrogenincorporation, and with additional base layers and emitter layerscomposed of GaAs, GaInAs, or low-nitrogen-content GaInNAs(Sb), havinglower N content and higher bandgap with respect to the base LBAR layer,forming improved transport (IT) layers in the base and emitter, due tothe absence or lower concentration of N.

FIG. 29 illustrates an example of a subcell within a solar cell, withalternating GaNAs(Sb) layers in the base in tensile strain, andGaInAs(Sb) layers in the base in compressive strain balancing the strainin the tensile-strain layers, such that both tensile and compressivelayers have lower bandgap than a GaAs or low-indium-content GaInAs baselayer, forming LBARs in both the tensile GaNAs(Sb) and compressiveGaInAs(Sb) layers in the base, and additionally forming improvedtransport (IT) layers in the compressive GaInAs(Sb) layers, due partlyto the absence of nitrogen.

FIG. 30 illustrates an example of a subcell within a solar cell, with anAlGa(In)As base, and with an Al-free Ga(In)As or low-Al-contentAlGa(In)As layer within the emitter and/or base, forming an LBAR and/orimproved transport (IT) layer, where the improved charge carriertransport properties may result from the absence or low concentration ofaluminum (Al) in the LBAR and/or IT layer. The example diagram shows acase in which the LBAR and IT layer is partly in the emitterquasi-neutral region and partly in the emitter space-charge region, andin which the LBAR and IT layer forms 100% of the emitter layer. The LBARand/or IT layer thickness may be varied to affect current balance in themultijunction cell, for example, the thickness may range from 0% to 100%of the total emitter thickness. In addition, the emitter thickness mayvary from 0% (no emitter case) to 100% (all emitter case) of thecombined emitter thickness plus the base thickness. In general, the LBARand/or IT layer or layers may have any of the configurations describedherein.

FIG. 31 illustrates an example of a subcell within a solar cell, with aGa(In)PAs base, and with an P-free Ga(In)As or low-P-content Ga(In)PAslayer within the emitter and/or base, forming an LBAR and/or improvedtransport (IT) layer, where the improved charge carrier transportproperties may result from the absence or low concentration ofphosphorus (P) in the LBAR and/or IT layer. The example diagram shows acase in which the LBAR and IT layer is partly in the emitterquasi-neutral region and partly in the emitter space-charge region, andin which the LBAR and IT layer forms 100% of the emitter layer. The LBARand/or IT layer thickness may be varied to affect current balance in themultijunction cell, for example, the thickness may range from 0% to 100%of the total emitter thickness. In addition, the emitter thickness mayvary from 0% (no emitter case) to 100% (all emitter case) of thecombined emitter thickness plus the base thickness. In general, the LBARand/or IT layer or layers may have any of the configurations describedherein.

FIG. 32 illustrates an example of a subcell within a solar cell, with aGa(In)(N)(P)As base, and with an Sb-containing Ga(In)(N)(P)AsSb layerwithin the emitter and/or base, forming an LBAR and/or improvedtransport (IT) layer, where the improved charge carrier transportproperties may result from the presence of antimony (Sb) in the LBARand/or IT layer, or during the growth of the LBAR and/or IT layer. Theexample diagram shows a case in which the LBAR and IT layer is partly inthe emitter quasi-neutral region and partly in the emitter space-chargeregion, and in which the LBAR and IT layer forms 100% of the emitterlayer. The LBAR and/or IT layer thickness may be varied to affectcurrent balance in the multijunction cell, for example, the thicknessmay range from 0% to 100% of the total emitter thickness. In addition,the emitter thickness may vary from 0% (no emitter case) to 100% (allemitter case) of the combined emitter thickness plus the base thickness.In general, the LBAR and/or IT layer or layers may have any of theconfigurations described herein.

FIG. 33 shows a diagram of a multijunction cell, showing features of ageneral multijunction solar cell structure. In the example shown in thefigure, the multijunction (MJ) cell is a 3-junction solar cell that hasa GaInP top cell (cell 1 or C1) base, a GaInAs cell 2 (C2) base, and aGe cell 3 (C3) base which also serves as a growth substrate or growthwafer. Also shown in the example in the figure is an optionalmetamorphic (MM) step-graded buffer. When such a metamorphic buffer isincluded, the component cells of the MJ cell (e.g., cell 1, cell 2, cell3, etc., also referred to as subcells) and layers grown after or on topof the metamorphic buffer are also referred to as metamorphic cells orlayers. When the component cells of the MJ cell are grown with the samematerial lattice constant as the growth substrate or as the subcell uponwhich they are grown (or with the same lattice constant due topseudomorphic strain) they are referred to as being lattice-matched (LM)to the growth substrate or subcell upon which they are grown, and theoptional metamorphic buffer is typically excluded in thislattice-matched case.

Referring again to FIG. 33, this figure illustrates a cross-sectionalview of a lattice-matched (LM) multijunction photovoltaic solar cell,and of a metamorphic (MM) multijunction (MJ) photovoltaic solar cell (MJcell) 10. The MJ cell 10 includes a top subcell 20, a middle subcell 40,and bottom subcell 60, connected in electrical series. The top, middleand bottom subcells 20, 40, 60 may be referred to according to thematerial of their respective base layers 24, 44, 64, or a GaInP subcell20, a GaInAs subcell 40, and a Ge subcell 60. The cell 10 may becomposed of a GaInP subcell 20 including a GaInP base layer 24, aGa(In)As subcell 40 including a Ga(In)As base layer 44 (where theparentheses around In indicates that In is an optional element), and aGe subcell 60 including a Ge base layer 64. The Ge base layer 64 isformed from a Ge growth substrate.

In another implementation, the cell 10 may be formed from: group III-Vsemiconductors, group IV semiconductors, group II-VI semiconductors,group I-III-VI semiconductors, and/or other semiconductor families. Inanother implementation, the cell 10 may be formed from semiconductormaterials selected from the group including GaAs, GaInAs, GaInP, AlGaAs,AlInAs, AlGaInAs, AlInP, AlGaInP, GaInPAs, AlInPAs, AlGaInPAs, GaPAs,InPAs, AlGaAsSb, AlInAsSb, GaInAsSb, GaAsSb, GaP, InP, AlAs, GaAs, InAs,AlSb, GaSb, InSb, GaNAs, GaInNAs, GaInNPAs, GaInNAsSb, AlGaInN, AlGaN,AlInN, GaInN, MN, GaN, InN, Ge, Si, SiGe, SiGeSn, SiC.

The top, middle and bottom subcells 20, 40, 60 may also be referred toby the order in which light strikes each subcell as it enters the cell10. Accordingly, the top subcell 20 may also be referred to as subcell1, the middle subcell 40 may be referred to as subcell 2, and the bottomsubcell 60 as subcell 3. In general, n subcells may be connected inseries, where n may be equal to 1 for a single junction cell, or n maybe any integer greater than or equal to 2 for a multijunction cell. Thegrowth substrate may be electrically inactive, or, it may beelectrically active, thereby forming one of the n subcells in themultijunction cell.

In one implementation, the cell 10 is a metamorphic (MM) MJ cell and themiddle cell 40 is a MM middle cell and the top cell 20 is a MM top cell.In another implementation, the cell 10 is a MM MJ cell and the middlecell 40 is a GaInAs middle cell and the top cell 20 is a MM GaInP topcell.

In one implementation, the Ge subcell 60 may be formed from a Ge waferthat serves as a substrate for epitaxial growth of the semiconductorlayers that form the upper subcells. The Ge wafer further serves as themain mechanical support for the cell, in addition to serving as one ofthe three active subcells in cell 10. The epitaxial growth ofsemiconductor layers on the substrate may be initiated with a nucleationlayer 58. The nucleation layer 58 can also serve as a window layer forthe Ge subcell 60.

A tunnel junction 47 is faulted atop the nucleation layer 58. The tunneljunction 47 includes a n⁺⁺ tunnel layer 48 and a p⁺⁺ tunnel layer 48.The tunnel junction 47 may be formed between the lowermost epitaxialsubcell and the above, beneath, or in the body of the metamorphic bufferregion 52.

The metamorphic buffer layer 52 includes 5 layers of transitioninglattice constant buffer layers between the bottom subcell 60 and themiddle subcell 40. In another implementation, the metamorphic bufferlayer 52 may contain one or more buffer layers. Such growth typicallyoccurs between the nucleation layer 58 and the lowermost epitaxialsubcell (such as the middle cell 40).

The bottom and middle subcells 60, 40 are lattice mismatched to oneanother, i.e., have a different lattice constant from one another.Additionally, the middle and top subcells 40, are lattice mismatched toone another. In an implementation, the cell 10 is a metamorphicstructure. As used herein, the term “lattice matched” means that thelattice constants are within 1% of each other. Also as used herein, theterm “lattice mismatched” means the lattice constants are different bymore than 1%.

In one implementation, the lattice constant of adjacent subcells differsby 0.5% or less. The difference between the lattice constants may alsobe in the range between 0.5% and 1.5%; between 1.5% and 2.5%; between2.5% and 4.5%; or may be greater than 4.5%.

In one implementation, the lattice constant of adjacent subcells isequal to or is within approximately 0.1% of the lattice constant ofGaAs, or 5.6533 angstroms. In another implementation, the latticeconstant of adjacent subcells is equal to or is within approximately0.1% of the lattice constant of Ge, or 5.6575 angstroms. In anotherimplementation, the value of the lattice constant of adjacent subcellsis equal to or is within approximately 0.1% of the lattice constant ofInP, or 5.8688 angstroms. In another implementation, the value of, thelattice constant of adjacent subcells is equal to or is withinapproximately 0.1% of the lattice constant of Si, or 5.4307 angstroms.In another implementation, the value of the lattice constant of adjacentsubcells is equal to or is within approximately 0.1% of the latticeconstant of GaSb, or 6.09593 angstroms. In another implementation, thelattice constant of adjacent subcells is equal to or is withinapproximately 0.1% of that of GaN with a wurtzite crystal latticestructure, characterized by lattice constants of 3.189 angstroms and5.185 angstroms. In another implementation, the lattice constant ofadjacent subcells is equal to or is within approximately 0.1% of thelattice constant of GaN with a zincblende crystal lattice structure, or4.50 angstroms. In another implementation, the lattice constant ofadjacent subcells is equal to or is within approximately 0.1% of thelattice constant of InAs, or 6.0584 angstroms. In anotherimplementation, the lattice constant of adjacent subcells is equal to oris within approximately 0.1% of the lattice constant of InSb, or 6.47937angstroms. In another implementation, the lattice constant of adjacentsubcells is equal to or is within approximately 0.1% of the latticeconstant of CdTe, or 6.482 angstroms.

In this exemplary implementation, the lattice constant is increasing inthe growth direction, or in other words, increasing in the directionfrom the bottom cell 60 toward the top cell 20 (the lattice constanttransition takes place in the metamorphic buffer between the bottom celland the middle cell). The increase in lattice constant in the growthdirection may be referred to as a grade in the compressive direction.

In another implementation, the lattice constant may decrease in thegrowth direction, or in other words, decreasing in the direction fromthe bottom cell 60 toward the top cell 20 (the lattice constanttransition takes place in the metamorphic buffer between the bottom celland the middle cell). The decrease in lattice constant in the growthdirection may be referred to as a grade in the tensile direction. Insuch an implementation, some material may change, for instance, themiddle cell 40 may be GaPAs instead of GaInAs, which allows the middlecell 40 to have a lattice constant smaller than that of GaAs.

The tunnel junction 27 connects the top subcell 20 and the middlesubcell 40 in electrical series, and the tunnel junction 47 connects themiddle subcell 40 and the bottom subcell 60 in electrical series. Ingeneral, each of the n subcells in a MJ cell, such as cell 10, may beconnected in series to the adjacent subcell(s) by a tunnel junction inorder to form a monolithic, two-terminal, series-interconnected MJ cell.In a two-terminal configuration it can be desirable to design thesubcell thicknesses and bandgaps such that each subcell has nearly thesame current at the maximum power point of the current-voltage curve ofeach subcell, in order that one subcell does not severely limit thecurrent of the other subcells. Alternatively, the top, middle and bottomsubcells 20, 40, 60 may be contacted by means of additional terminals,for instance, metal contacts to laterally conductive semiconductorlayers between the subcells, to form 3-terminal, 4-terminal, and ingeneral, m-terminal MJ cells (m being an integer greater than or equalto 2, and less than or equal to 2n, where n is the number of activesubcells in the MJ cell). The top, middle and bottom subcells 20, 40, 60may be interconnected in circuits using these additional terminals suchthat most of the available photogenerated current density in eachsubcell can be used effectively. Such effective use may lead to highefficiency for the cell 10, even if the photogenerated current densitiesare very different in the various subcells.

A window 21, emitter 22, base 24, and back-surface field (BSF) layer 25is shown in the top cell 20, a window 41, emitter 42, base 44 and BSFlayer 45 are shown in the middle cell 40, and an emitter 62 and base 63are shown the bottom cell 60.

A variety of different semiconductor materials may be used for thewindow layers 21, 41, and the buffer layer 52 and the nucleation layer58. The buffer layer 52 and nucleation layer 58 also serve as windowlayers for the bottom cell 60. The variety of different semiconductormaterials may be used for the window layers 21, 41, and the buffer layer52 and the nucleation layer 58 may include AlInP, AlAs, AlP, AlGaInP,AlGaAsP, AlGaInAs, AlGaInPAs, GaInP, GaInAs, GaInPAs, AlGaAs, AlInAs,AlInPAs, GaAsSb, AlAsSb, GaAlAsSb, AlInSb, GaInSb, AlGaInSb, AlN, GaN,InN, GaInN, AlGaInN, GaInNAs, AlGaInNAs, Ge, Si, SiGe, ZnSSe, CdSSe, andother materials and still fall within the spirit of the presentdisclosure.

The emitter layers 22, 42, 62 may typically be thinner than thecorresponding base layers 24, 44, 64 and positioned on the sunward sideof the base layers, though some specialized cells also make use of backsurface illumination incident on the back of the base. Most of thephotogeneration of electron-hole pairs responsible for the cell currenttypically takes place in the base layers, though the photogeneratedcurrent density from the emitter layers 22, 42, 62 can also besignificant in most cells, and in some specialized cells may exceed thatin the base layers 24, 44, 64.

The emitter layer 62 of the Ge subcell 60 can be formed by diffusioninto the p-type Ge substrate of column-V elements (which are n-typedopants in Ge) from the epitaxial growth of the III-V semiconductors ontop of the Ge substrate. The base 64 of the Ge subcell 60 consists ofthe bulk of the p-type Ge wafer which also serves as the growthsubstrate and mechanical support for the rest of the cell 10. Although,no BSF layer appears on the back of the Ge subcell 60, a BSF layer (suchas a diffused p+ region, or an epitaxially-grown group-IV or III-Vsemiconductor layer) may be positioned in such a location to improve theefficiency of the Ge subcell 60, as well as the overall cell 10efficiency.

Additionally, while the base layer 64 and growth substrate maypreferably be a p-Ge base layer 64 and growth substrate, othersemiconductor materials may be used as the base layer 64 and growthsubstrate, or only as a growth substrate. These include, but are notlimited to, GaAs, InP, GaSb, InAs, InSb, GaP, Si, SiGe, SiC, Al₂O₃, Mo,stainless steel, soda-lime glass, and SiO₂.

The cap layer 15 is disposed upon the top cell 20. The cap layer 15 is asemiconductor layer that fortes a low resistance electrical contact tothe top surface of the top cell 20, i.e., to the window of the top cell,and also forms a low resistance electrical contact to the metalelectrode or grid pattern on the top of the cell, in order for currentto be conducted from the top cell to the metal electrode which forms thetop electrical terminal of the solar cell with a minimum of resistivepower loss. It is typically heavily doped, designated by n⁺-doping orp⁺-doping, in order to achieve low contact resistance to the metalelectrode. The cap also serves to separate the active top cell layersfrom the metal layers in the top electrode, which can have deleteriouseffects if those metals are allowed to diffuse into or otherwise enterthe active top cell layers. The cap layer 15 may be a n⁺-doped GaInAslayer. In other implementations, the cap layer 15 may be composed ofGaAs, GaInAs, GaNAs, GaInNAs, GaAsSb, AlGaAs, AlGaInAs, GaPAs, GaInP,GaInPAs, Ge, SiGe, and other III-V or group-IV semiconductors, andcombinations of these materials. In other implementations, the cap layer15 may have very high n-type doping, designated n⁺-doping, or very highp-type doping, designated p⁺-doping, where very high doping typicallyindicates doping >10¹⁸ cm⁻³, and preferably >10¹⁹ cm⁻³, and morepreferably >10²⁰ cm⁻³, particularly at the interface with the metalelectrode, in order to minimize contact resistance to the metalelectrode. In other implementations, the cap layer 15 may comprisemultiple layers, each of which may have a different function in the cap,such a cap comprising a heavily-doped layer near the metal interface anda less heavily-doped layer contacting the top cell window; a capcomprising one or more layers used as a lateral conductance layer; a capcomprising one or more metal diffusion barrier layers; a cap comprisingone or more strained layers, e.g., to balance strain elsewhere in thecell, to create strain elsewhere in the cell, or to achieve a lowerbandgap to make low resistance contact formation easier; and a capcomprising one or more high bandgap layers or thinned layers tofacilitate light transmission through the cap into the solar cell.

The photogenerated current leaves the respective subcell throughcontacting layers, which are typically heavily-doped semiconductorlayers, but may be composed of other types of conductive material, suchas conductive oxides or metal, which may be transparent or opaque overdifferent wavelength ranges. The contacting layers for the top subcell20 are the cap layer 18 on the front of the subcell 20 (which in turn iscontacted by the metal grid pattern 14 on the top of the cell 10), andthe p⁺⁺-doped side 28 of the tunnel junction 27 on the back surface ofthe top subcell 20. The contacting layers for the middle subcell 40 arethe n⁺⁺-doped side 29 of the tunnel junction 27 on front of the middlesubcell 40, and the p⁺⁺-doped side 48 of the tunnel junction 47 on theback surface of the middle subcell 40. The contacting layers for the Gebottom subcell 60 are the n⁺⁺-doped side 49 of the tunnel junction 47 onfront of the buffer region 52 (provided that the buffer region 52 isconsidered to be part of the window structure for the Ge subcell 60),and the back metal contact 68 on the back surface of the bottom subcell60 (which can also be considered the back surface of the entire cell10). These contacting layers may be unpatterned, as in the case of theback metal contact 68 on the bottom subcell 60, or a transparentconductive oxide contacting the top cell window 21 or emitter 22, inplace of a more conventional solar cell grid. The contacting layers mayalso be patterned, as in the case of the patterned heavily-doped cap 18and metal contact 14 that form the front grid of most solar cells. Ananti-reflection coating 16 can be provided on the PV cell's 10 front(sunward) surface (and, for example, disposed above the AlInP windowlayer 21), and may be typically made up of one, two, or more dielectriclayers with thicknesses optimized to maximize transmission of lightthrough the front surface over the range of wavelengths to which the PVcell can be responsive.

The lateral conductivity of the emitter and window layers betweengridlines can be important, since after minority carriers in the base(minority electrons in the case of the p-type top cell base 24) arecollected at the base/emitter p-n junction between the gridlines, thecollected carriers, which are now majority carriers in the emitter(majority electrons in the n-type top cell emitter 22), must beconducted to the gridlines with minimum resistive loss. Both the topcell emitter layer 22 and window layer 21 take part in this lateralmajority-carrier conduction to the gridlines. While maintaining thishigh conductivity, the window 21 and emitter layers 22 should remainhighly transmissive to photon energies that can be used effectively bythe base 24 of the top cell 20 and by the other active subcells 40, 60in the cell 10. Further, the window 21 and emitter layers 22 should havea long diffusion length for minority-carriers that are photogenerated inthe window 21 and emitter layer 22 (minority holes in the case of then-type emitter 22), so that they may be collected at the p-n junctionbefore recombining. Since the transmittance and diffusion length bothtend to decrease for high doping levels, an optimum doping leveltypically exists at which cell efficiency can be maximized, for whichthe conductivity of the window 21 and emitter layer 22 can be highenough that resistive losses are small compared to the power output ofthe cell 20, and yet the transmittance and minority-carrier collectionin the window 21 and emitter layer 22 are high enough that most of thephotons incident on the cell 20 generate useful current.

The highly-doped layers that form the tunnel junctions between cells,with their very low sheet resistance, also serve as lateral conductionlayers. Such conduction layers help to make the current density acrossthe cell 10 more uniform in the case of spatially non-uniform intensityor spectral content of the light incident on the cell.Laterally-conductive layers between the subcells 20, 40, and on the backof the bottom cell 60, are also very important in the case of MJ celldesigns which have more than two terminals. For example, inmechanically-stacked or monolithically-grown MJ cells, with 3, 4, ormore terminals, the subcells operate at current densities that are notall necessarily the same in order to optimize the efficiency of eachsubcell and hence of the entire MJ cell. Laterally-conductive regionsbetween the subcells 20, 40 and at the back of the bottom cell 60 arealso important for configurations with 3, 4, or more terminals in whichthe subcells are interconnected with other circuit elements, such asbypass or blocking diodes, or in which the subcells from one MJ cell areconnected with subcells in another MJ cell, in series, in parallel, orin a combination of series and parallel, in order to improve theefficiency, voltage stability, or other performance parameter of thephotovoltaic cell circuit.

FIGS. 34-35 illustrate examples of a 3-junction solar cell (FIG. 34) anda 4-junction solar cell (FIG. 35), incorporating a GaInP orlow-Al-content AlGaInP emitter LBAR or improved transport (IT) layer anda GaInP or AlGaInP base in cell 1, i.e., the top subcell of themultijunction cell. Preferred bandgap ranges for the subcells are givenin the diagram, though the subcells may have bandgaps outside of theseranges. Some possible semiconductor compositions for the mainphotoabsorbing layer in each subcell are shown, but other compositionsmay be used. The LBAR or IT layers incorporated in the multijunctioncells may be in any of the subcells, and may be of any of the typesdescribed elsewhere herein. In addition, the layers 10 can be tunneljunction layers between subcells, and optional graded buffer layers inIMM cells, or bonding layers in transparent layer/metal bonded orsemiconductor bonded cells.

FIGS. 36-37 illustrate examples of a 5-junction solar cell (FIG. 36) anda 6-junction solar cell (FIG. 37), incorporating a GaInP orlow-Al-content AlGaInP emitter LBAR or improved transport (IT) layer anda GaInP or AlGaInP base in cell 1, i.e., the top subcell of themultijunction cell. Preferred bandgap ranges for the subcells are givenin the diagram, though the subcells may have bandgaps outside of theseranges. Some possible semiconductor compositions for the mainphotoabsorbing layer in each subcell are shown, but other compositionsmay be used. The LBAR or IT layers incorporated in the multijunctioncells may be in any of the subcells, and may be of any of the typesdescribed elsewhere herein. In addition, the layers 10 can be tunneljunction layers between subcells, and optional graded buffer layers inIMM cells, or bonding layers in transparent layer/metal bonded orsemiconductor bonded cells.

FIGS. 38-40 illustrate examples of a 7-junction solar cell (FIG. 38), an8-junction solar cell (FIG. 39), and a 9-junction solar cell (FIG. 40),incorporating a GaInP or low-Al-content AlGaInP emitter LBAR or improvedtransport (IT) layer and a GaInP or AlGaInP base in cell 1, i.e., thetop subcell of the multijunction cell. Preferred bandgap ranges for thesubcells are given in the diagram, though the subcells may have bandgapsoutside of these ranges. Some possible semiconductor compositions forthe main photoabsorbing layer in each subcell are shown, but othercompositions may be used. The LBAR or IT layers incorporated in themultijunction cells may be in any of the subcells, and may be of any ofthe types described elsewhere herein. In addition, the layers 10 can betunnel junction layers between subcells, and optional graded bufferlayers in IMM cells, or bonding layers in transparent layer/metal bondedor semiconductor bonded cells.

FIGS. 41-42 plot the measured external quantum efficiency of solar cellswith a 10%-Al AlGaInP top cell base, and a 10%-Al AlGaInP top cellemitter, with no LBAR in the emitter (control case) (FIG. 41), and anAl-free GaInP (0%-Al AlGaInP) top cell emitter forming a low bandgapabsorber region (LBAR) in the emitter (LBAR case) (FIG. 42) that isprimarily in the quasi-neutral region of the emitter. The thickness ofthe LBAR in this experiment is approximately 750 angstroms, such thatthe LBAR thickness is significantly greater than the energy wellthickness (˜200 angstroms) below which the separation of electron andhole energy levels is significantly increased due to quantum mechanicaleffects. The LBAR in the emitter of the LBAR case benefits the currentcollected in both the short wavelength and long wavelength regions ofthe external quantum efficiency (EQE) curves for thesenon-anti-reflection-coated top cells. Compared to the control case with˜42% EQE at 400 nm, ˜6% EQE at 640 nm, and ˜0% EQE at 660 nm, while theLBAR case exhibits ˜52% EQE at 400 nm, ˜12% EQE at 640 nm, and 5% EQE at660 nm, due to the photogeneration of charge carriers at lower photonenergies by the LBAR and the improved transport (IT) of minoritycarriers (minority holes in this case of an n-type emitter) due to theAl-free composition of the emitter LBAR.

FIG. 43 plots the measured external quantum efficiency of solar cellswith 2.05-eV AlGaInP top cell bases, and with 3 different LBAR cases inthe top cell emitter: 1) a homojunction 2.05-eV AlGaInP emitter with noLBAR in the emitter (control case); 2) a low-Al 1.95-eV AlGaInP emitterLBAR (low-Al case); and 3) an Al-free 1.88-eV GaInP emitter LBAR(Al-free case). The LBAR in the emitter for both the low-Al and Al-freecases benefits the current collected in both the short wavelength andlong wavelength regions of the external quantum efficiency (EQE) curvesfor these non-anti-reflection-coated top cells. Compared to the controlcase with ˜46% EQE at 400 nm and ˜4% EQE at 625 nm, the low-Al case has˜52% EQE at 400 nm and ˜4% EQE at 625 nm, while the Al-free emitter LBARcase has ˜55% at 400 and ˜9% EQE at 625 nm, due to the photogenerationof charge carriers at lower photon energies by the LBAR and the improvedtransport (IT) of minority carriers (minority holes in this case of ann-type emitter) due to the Al-free or low-Al composition of the emitterLBAR.

Various implementations of the disclosure have been described infulfillment of the various objectives of the disclosure. It should berecognized that these implementations are merely illustrative of theprinciples of the present disclosure. Numerous modifications andadaptations thereof will be readily apparent to those skilled in the artwithout departing from the spirit and scope of the disclosure.

That which is claimed is:
 1. An optoelectronic device comprising aphotovoltaic cell, the photovoltaic cell comprising: a space-chargeregion; a quasi-neutral region; and a low bandgap absorber region (LBAR)layer or an improved transport (IT) layer at least partially positionedin the quasi-neutral region of the cell.
 2. The device of claim 1,wherein the LBAR layer has a lower bandgap than an immediately adjacentsemiconductor layer of the cell.
 3. The device of claim 1, wherein theLBAR layer has a lower bandgap than two immediately adjacentsemiconductor layers of the cell.
 4. The device of claim 1, wherein theIT layer has a higher collected photogenerated current density than areplacement layer having the composition of an immediately adjacentphotogeneration layer.
 5. The device of claim 1, wherein the IT layerhas a higher collected photogenerated current density than a replacementlayer having the composition of either of the two immediately adjacentphotogeneration layers.
 6. The device of claim 1, wherein the LBAR layeror the IT layer is positioned entirely in the quasi-neutral region ofthe cell.
 7. The device of claim 1, wherein the LBAR layer or the ITlayer is positioned partly in the quasi-neutral region of the cell andpartly in the space-charge region of the cell.
 8. The device of claim 1,wherein the cell further comprises an emitter layer and the LBAR layeror the IT layer is positioned at least partially in the quasi-neutralregion of the emitter layer.
 9. The device of claim 1, wherein the cellfurther comprises a base layer and the LBAR layer or the IT layer ispositioned at least partially in the quasi-neutral region of the baselayer.
 10. The device of claim 1, wherein the LBAR layer or the IT layerforms the entirety of a functional layer of the cell.
 11. The device ofclaim 10, wherein the functional layer comprises an emitter layer, baselayer, window layer, or back-surface-field (BSF) layer.
 12. The deviceof claim 10, wherein the functional layer comprises an emitter layer.13. The device of claim 12, wherein the emitter layer has a thicknessthat is between about 50% and about 100% of the total thickness of thephotoabsorber of the cell.
 14. The device of claim 12, wherein the cellfurther comprises a base layer and wherein the emitter layer has athickness that is up to about 20% of the sum of the emitter layerthickness plus the base layer thickness.
 15. The device of claim 1,wherein the cell comprises an LBAR layer and an IT layer at leastpartially positioned in the quasi-neutral region.
 16. The device ofclaim 15, wherein the LBAR layer and the IT layer are the same layer.17. The device of claim 12, wherein the cell further comprises: a windowlayer; a BSF layer; and a base layer, wherein the base layer and theemitter layer form a photoabsorber layer disposed between the windowlayer and the BSF layer.
 18. The device of claim 1, wherein the LBARlayer or the IT layer forms a first portion of an emitter layer of thecell and the cell further comprises: a window layer; aback-surface-field (BSF) layer; a base layer; and a second portion ofthe emitter layer, wherein the first portion of the emitter layer isdisposed between the base layer and the second portion of the emitterlayer, and the first portion of the emitter layer is positionedpartially in the space-charge region of the emitter layer and partiallyin the quasi-neutral region of the emitter layer.
 19. The device ofclaim 1, wherein the LBAR layer or the IT layer forms a first portion ofan emitter layer of the cell and the cell further comprises: a windowlayer; a back-surface-field (BSF) layer; a base layer; and a secondportion of the emitter layer, wherein the second portion of the emitterlayer is disposed between the base layer and the first portion of theemitter layer, the first portion of the emitter layer is adjacent thewindow layer, and the first portion of the emitter layer is positionedentirely in the quasi-neutral region of the emitter layer.
 20. Thedevice of claim 1, wherein the LBAR layer or the IT layer forms a firstportion of a base layer of the cell and the cell further comprises: awindow layer; a back-surface-field (BSF) layer; an emitter layer; and asecond portion of the base layer, wherein the first portion of the baselayer is disposed between the emitter layer and the second portion ofthe base layer, the second portion of the base layer is adjacent the BSFlayer, and the first portion of the base layer is positioned partiallyin the space-charge region of the base layer and partially in thequasi-neutral region of the base layer.
 21. The device of claim 1,wherein the LBAR layer or the IT layer forms a first portion of a baselayer of the cell and the cell further comprises: a window layer; aback-surface-field (BSF) layer; an emitter layer; a second portion ofthe base layer; and a third portion of the base layer, wherein the firstportion of the base layer is disposed between the second and thirdportions of the base layer, and the first portion of the base layer ispositioned partially in the space-charge region of the base layer andpartially in the quasi-neutral region of the base layer.
 22. The deviceof claim 1, wherein the LBAR layer or the IT layer forms a first portionof a base layer of the cell and the cell further comprises: a windowlayer; a back-surface-field (BSF) layer; an emitter layer; and a secondportion of the base layer, wherein the first portion of the base layeris disposed between the BSF layer and the second portion of the baselayer, the second portion of the base layer is adjacent the emitterlayer, and the first portion of the base layer is positioned entirely inthe quasi-neutral region of the base layer.
 23. The device of claim 1,wherein the LBAR layer or the IT layer forms a first portion of a baselayer of the cell and the cell further comprises: a window layer; aback-surface-field (BSF) layer; an emitter layer; a second portion ofthe base layer; and a third portion of the base layer, wherein the firstportion of the base layer is disposed between the second portion of thebase layer and the third portion of the base layer, the second portionof the base layer is adjacent the emitter layer, the third portion ofthe base layer is disposed between the first portion of the base layerand the BSF layer, and the first portion of the base layer is positionedentirely in the quasi-neutral region of the base layer.
 24. The deviceof claim 1, wherein the LBAR layer or the IT layer is free orsubstantially free of aluminum and at least one semiconductor layerimmediately adjacent the LBAR layer or the IT layer includes aluminumand has a higher bandgap than the LBAR layer or the IT layer.
 25. Thedevice of claim 1, wherein the LBAR layer or the IT layer comprises nomore than about 15 mole percent aluminum relative to the total amount ofgroup III elements present in the layer and wherein at least onesemiconductor layer immediately adjacent the LBAR layer or the IT layerincludes a higher mole percent of aluminum than the LBAR layer or the ITlayer and has a higher bandgap than the LBAR layer or the IT layer. 26.The device of claim 1, wherein the cell further comprises an emitterlayer and a base layer, and wherein the LBAR layer or the IT layercomprises no more than about 15 mole percent aluminum relative to thetotal amount of group III elements present in the layer, and ispositioned in or composes the entirety of the emitter layer, and whereinthe base layer includes a higher mole percent of aluminum than the LBARlayer or the IT layer and has a higher bandgap than the LBAR layer orthe IT layer.
 27. The device of claim 1, wherein the LBAR layer or theIT layer forms a first portion of a base layer of the cell and the cellfurther comprises: a window layer; a back-surface-field (BSF) layer; anemitter layer; a second portion of the base layer; and a third portionof the base layer, wherein the first portion of the base layer isdisposed between the second portion of the base layer and the thirdportion of the base layer, the second portion of the base layer isadjacent the emitter layer, the third portion of the base layer isdisposed between the first portion of the base layer and the BSF layer,and the first portion of the base layer is positioned entirely in thequasi-neutral region of the base layer, entirely in the space-chargeregion, or partly in the quasi-neutral region and partly in thespace-charge region, wherein the first portion of the base layer has alower aluminum mole percent and lower bandgap than the second and thirdportions of the base layer.
 28. The device of claim 1, wherein the LBARlayer or the IT layer forms a first portion of a base layer of the celland the cell further comprises: a window layer; a back-surface-field(BSF) layer; an emitter layer; a second portion of the base layer; and athird portion of the base layer, wherein the first portion of the baselayer is disposed between the second portion of the base layer and thethird portion of the base layer, the second portion of the base layer isadjacent the emitter layer, the third portion of the base layer isdisposed between the first portion of the base layer and the BSF layer,and wherein the first portion of the base layer has a greater degree ofgroup-III sublattice ordering and a lower bandgap than the second andthird portions of the base layer.
 29. The device of claim 1, wherein theLBAR layer or the IT layer comprises one or more layers of a first layertype having a first bandgap and a first amount of strain with respect tothe average lattice constant of the photovoltaic cell, and furthercomprises one or more layers of a second layer type having a secondbandgap that is greater than the first bandgap, and a second amount ofstrain such that the layers of the second layer type are in tensilestrain with respect to the layers of the first layer type, and whereinthe strain of the layers of the first layer type is balanced with thestrain of the layers of the second layer type such that both types oflayers remain pseudomorphic and retain a coherent lattice structure witha crystal defect areal density lower than about 10⁶ cm⁻².
 30. The deviceof claim 1, wherein the LBAR layer or the IT layer includes nitrogen andat least one semiconductor layer immediately adjacent the LBAR layer orthe IT layer is free or substantially free of nitrogen and has a higherbandgap than the LBAR layer or the IT layer.
 31. The device of claim 1,wherein the LBAR layer or the IT layer includes nitrogen and at leastone semiconductor layer immediately adjacent the LBAR layer or the ITlayer includes a non-zero amount of nitrogen, the immediately adjacentsemiconductor layer having a higher bandgap than the LBAR layer or theIT layer and a lower mole percent of nitrogen than the LBAR layer or theIT layer.
 32. The device of claim 1, wherein the cell further comprises:an emitter layer; and a base layer having a higher bandgap than theemitter layer, wherein an LBAR layer including nitrogen is disposed inthe emitter layer, and the base layer has a lower nitrogen content thanthe LBAR layer and forms an IT layer.
 33. The device of claim 1, whereinthe cell further comprises: a base layer; and an emitter layer having ahigher bandgap than the base layer, wherein an LBAR layer includingnitrogen is disposed in the base layer and an emitter layer having ahigher bandgap and a lower nitrogen content than the LBAR layer forms anIT layer.
 34. The device of claim 1, wherein the LBAR layer or the ITlayer comprises one or more of GaInP, AlGaInP, GaAs, AlGaAs, GaInAs,GaAsSb, AlGaInAs, AlGaAsSb, GaInPAs, and AlGaInPAs.
 35. The device ofclaim 1, wherein the LBAR layer or the IT layer comprises one or more ofGaNAs, GaInNAs, GaNAsSb, and GaInNAsSb.
 36. The device of claim 1,wherein the device is an inorganic semiconductor device.
 37. The deviceof claim 1, wherein the device comprises a plurality of photovoltaiccells.
 38. The device of claim 37, wherein the device comprises a multijunction photovoltaic device and each photovoltaic cell forms a subcellof the photovoltaic device.
 39. The device of claim 38, wherein at leasttwo of the cells of the device comprise: a space-charge region; aquasi-neutral region; and a low bandgap absorber region (LBAR) layer oran improved transport (IT) layer at least partially positioned in thequasi-neutral region of the cell.
 40. The device of claim 1, wherein thedevice comprises a photodiode.